diff --git a/CLAUDE.md b/CLAUDE.md
index 40d6008..265d2b9 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -29,3 +29,11 @@ PID=$(netstat -ano 2>/dev/null | grep ':5173.*LISTENING' | awk '{print $5}' | he
 - **System-owned entities**: `user_id=0` means system-owned (e.g. default templates).
 - **FastAPI route ordering**: Static path routes MUST be registered BEFORE parameterized routes.
 - **`__pycache__`**: Add to `.gitignore`. Never commit.
+
+## Project Structure (Phase 1)
+
+- **packages/core** (`notify_bridge_core`): Shared library — providers, models, notifications, templates. No DB dependency.
+- **packages/server** (`notify_bridge_server`): FastAPI REST API + SQLite. Depends on core.
+- **frontend**: SvelteKit 2 + Svelte 5 + Tailwind CSS v4. Static adapter with SPA fallback. Dev proxy to :8420.
+- **Environment vars**: `NOTIFY_BRIDGE_DATA_DIR`, `NOTIFY_BRIDGE_SECRET_KEY`, `NOTIFY_BRIDGE_DATABASE_URL`
+- Core package includes `jinja2` dependency (template rendering lives in core, not server).
diff --git a/packages/core/src/notify_bridge_core/models/__init__.py b/packages/core/src/notify_bridge_core/models/__init__.py
index c5f086b..364238c 100644
--- a/packages/core/src/notify_bridge_core/models/__init__.py
+++ b/packages/core/src/notify_bridge_core/models/__init__.py
@@ -1 +1,14 @@
 """Core data models — events, media assets, collections."""
+
+from notify_bridge_core.models.collections import CollectionState
+from notify_bridge_core.models.events import EventType, ServiceEvent
+from notify_bridge_core.models.media import MediaAsset, MediaCollection, MediaType
+
+__all__ = [
+    "CollectionState",
+    "EventType",
+    "MediaAsset",
+    "MediaCollection",
+    "MediaType",
+    "ServiceEvent",
+]
diff --git a/packages/core/src/notify_bridge_core/models/collections.py b/packages/core/src/notify_bridge_core/models/collections.py
new file mode 100644
index 0000000..aad677e
--- /dev/null
+++ b/packages/core/src/notify_bridge_core/models/collections.py
@@ -0,0 +1,18 @@
+"""Collection state tracking for change detection."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from datetime import datetime
+
+
+@dataclass
+class CollectionState:
+    """Persisted state for a tracked collection, used for change detection."""
+
+    collection_id: str
+    asset_ids: set[str] = field(default_factory=set)
+    pending_asset_ids: set[str] = field(default_factory=set)
+    name: str = ""
+    shared: bool = False
+    last_updated: datetime | None = None
diff --git a/packages/core/src/notify_bridge_core/models/events.py b/packages/core/src/notify_bridge_core/models/events.py
new file mode 100644
index 0000000..ad7438c
--- /dev/null
+++ b/packages/core/src/notify_bridge_core/models/events.py
@@ -0,0 +1,54 @@
+"""Generic event models for service provider notifications."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from datetime import datetime
+from enum import Enum
+from typing import Any
+
+from notify_bridge_core.models.media import MediaAsset
+from notify_bridge_core.providers.base import ServiceProviderType
+
+
+class EventType(str, Enum):
+    """Types of events a service provider can emit."""
+
+    ASSETS_ADDED = "assets_added"
+    ASSETS_REMOVED = "assets_removed"
+    COLLECTION_RENAMED = "collection_renamed"
+    COLLECTION_DELETED = "collection_deleted"
+    SHARING_CHANGED = "sharing_changed"
+
+
+@dataclass
+class ServiceEvent:
+    """A generic event emitted by a service provider.
+
+    Contains all information needed to render a notification template.
+    Provider-specific data goes in the `extra` dict.
+    """
+
+    event_type: EventType
+    provider_type: ServiceProviderType
+    provider_name: str
+    collection_id: str
+    collection_name: str
+    timestamp: datetime
+
+    # Change details
+    added_assets: list[MediaAsset] = field(default_factory=list)
+    removed_asset_ids: list[str] = field(default_factory=list)
+    added_count: int = 0
+    removed_count: int = 0
+
+    # For renames
+    old_name: str | None = None
+    new_name: str | None = None
+
+    # For sharing changes
+    old_shared: bool | None = None
+    new_shared: bool | None = None
+
+    # Provider-specific extra data
+    extra: dict[str, Any] = field(default_factory=dict)
diff --git a/packages/core/src/notify_bridge_core/models/media.py b/packages/core/src/notify_bridge_core/models/media.py
new file mode 100644
index 0000000..fdbedac
--- /dev/null
+++ b/packages/core/src/notify_bridge_core/models/media.py
@@ -0,0 +1,56 @@
+"""Generic media asset and collection models."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from datetime import datetime
+from enum import Enum
+from typing import Any
+
+
+class MediaType(str, Enum):
+    """Types of media assets."""
+
+    IMAGE = "image"
+    VIDEO = "video"
+    DOCUMENT = "document"
+
+
+@dataclass
+class MediaAsset:
+    """A generic media asset from a service provider.
+
+    Common fields are top-level. Provider-specific data goes in `extra`.
+    """
+
+    id: str
+    type: MediaType
+    filename: str
+    created_at: datetime
+
+    # Common optional fields
+    owner_name: str | None = None
+    description: str | None = None
+    tags: list[str] = field(default_factory=list)
+    thumbnail_url: str | None = None
+    full_url: str | None = None
+
+    # Provider-specific extras (e.g., rating, location, people for Immich)
+    extra: dict[str, Any] = field(default_factory=dict)
+
+
+@dataclass
+class MediaCollection:
+    """A generic collection of media assets (album, folder, etc.).
+
+    Provider-specific data goes in `extra`.
+    """
+
+    id: str
+    name: str
+    asset_count: int = 0
+    assets: dict[str, MediaAsset] = field(default_factory=dict)
+    asset_ids: set[str] = field(default_factory=set)
+
+    # Provider-specific extras
+    extra: dict[str, Any] = field(default_factory=dict)
diff --git a/packages/core/src/notify_bridge_core/providers/__init__.py b/packages/core/src/notify_bridge_core/providers/__init__.py
index bce70fc..4aefed6 100644
--- a/packages/core/src/notify_bridge_core/providers/__init__.py
+++ b/packages/core/src/notify_bridge_core/providers/__init__.py
@@ -1 +1,5 @@
 """Service provider abstractions and implementations."""
+
+from notify_bridge_core.providers.base import ServiceProvider, ServiceProviderType
+
+__all__ = ["ServiceProvider", "ServiceProviderType"]
diff --git a/packages/core/src/notify_bridge_core/providers/base.py b/packages/core/src/notify_bridge_core/providers/base.py
new file mode 100644
index 0000000..a72d782
--- /dev/null
+++ b/packages/core/src/notify_bridge_core/providers/base.py
@@ -0,0 +1,76 @@
+"""Service provider abstraction — base class and types."""
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from enum import Enum
+from typing import TYPE_CHECKING, Any
+
+if TYPE_CHECKING:
+    from notify_bridge_core.models.events import ServiceEvent
+    from notify_bridge_core.templates.variables import TemplateVariableDefinition
+
+
+class ServiceProviderType(str, Enum):
+    """Supported service provider types."""
+
+    IMMICH = "immich"
+
+
+class ServiceProvider(ABC):
+    """Abstract base class for service providers.
+
+    A service provider connects to an external service (e.g., Immich photo server)
+    and can poll for changes, producing generic ServiceEvent objects.
+    """
+
+    provider_type: ServiceProviderType
+
+    @abstractmethod
+    async def connect(self) -> bool:
+        """Connect to the service and verify connectivity.
+
+        Returns True if connection is successful.
+        """
+
+    @abstractmethod
+    async def disconnect(self) -> None:
+        """Disconnect and clean up resources."""
+
+    @abstractmethod
+    async def poll(
+        self,
+        collection_ids: list[str],
+        tracker_state: dict[str, Any],
+    ) -> tuple[list[ServiceEvent], dict[str, Any]]:
+        """Poll for changes in the specified collections.
+
+        Args:
+            collection_ids: IDs of collections to check.
+            tracker_state: Previous state dict (opaque to caller, managed by provider).
+
+        Returns:
+            Tuple of (list of events detected, updated state dict).
+        """
+
+    @abstractmethod
+    def get_available_variables(self) -> list[TemplateVariableDefinition]:
+        """Return the template variables this provider makes available."""
+
+    @abstractmethod
+    def get_provider_config_schema(self) -> dict[str, Any]:
+        """Return JSON schema for this provider's configuration fields."""
+
+    @abstractmethod
+    async def list_collections(self) -> list[dict[str, Any]]:
+        """List available collections from the service.
+
+        Returns a list of dicts with at least 'id' and 'name' keys.
+        """
+
+    @abstractmethod
+    async def test_connection(self) -> dict[str, Any]:
+        """Test the connection and return status info.
+
+        Returns a dict with 'ok' (bool) and optional 'message' (str).
+        """
diff --git a/packages/core/src/notify_bridge_core/templates/__init__.py b/packages/core/src/notify_bridge_core/templates/__init__.py
index 61ad195..8e43061 100644
--- a/packages/core/src/notify_bridge_core/templates/__init__.py
+++ b/packages/core/src/notify_bridge_core/templates/__init__.py
@@ -1 +1,15 @@
 """Template system — rendering, variables, validation."""
+
+from notify_bridge_core.templates.variables import (
+    BASE_VARIABLES,
+    TemplateVariableDefinition,
+    VariableRegistry,
+    registry,
+)
+
+__all__ = [
+    "BASE_VARIABLES",
+    "TemplateVariableDefinition",
+    "VariableRegistry",
+    "registry",
+]
diff --git a/packages/core/src/notify_bridge_core/templates/variables.py b/packages/core/src/notify_bridge_core/templates/variables.py
new file mode 100644
index 0000000..a0aabe9
--- /dev/null
+++ b/packages/core/src/notify_bridge_core/templates/variables.py
@@ -0,0 +1,135 @@
+"""Template variable system — definitions and registry."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+from notify_bridge_core.providers.base import ServiceProviderType
+
+
+@dataclass
+class TemplateVariableDefinition:
+    """Definition of a variable available in notification templates."""
+
+    name: str
+    type: str  # "string", "int", "list", "datetime", "bool"
+    description: str
+    example: str
+    provider_type: ServiceProviderType | None = None  # None = base variable
+
+
+# Base variables available to all providers
+BASE_VARIABLES: list[TemplateVariableDefinition] = [
+    TemplateVariableDefinition(
+        name="event_type",
+        type="string",
+        description="Event type identifier (e.g., 'assets_added')",
+        example="assets_added",
+    ),
+    TemplateVariableDefinition(
+        name="timestamp",
+        type="datetime",
+        description="When the event occurred",
+        example="2026-03-19 14:30:00",
+    ),
+    TemplateVariableDefinition(
+        name="service_name",
+        type="string",
+        description="Name of the service provider instance",
+        example="My Immich Server",
+    ),
+    TemplateVariableDefinition(
+        name="service_type",
+        type="string",
+        description="Type of service provider",
+        example="immich",
+    ),
+    TemplateVariableDefinition(
+        name="collection_name",
+        type="string",
+        description="Name of the collection (album, folder, etc.)",
+        example="Vacation 2026",
+    ),
+    TemplateVariableDefinition(
+        name="collection_id",
+        type="string",
+        description="Unique identifier of the collection",
+        example="abc-123-def",
+    ),
+    TemplateVariableDefinition(
+        name="added_count",
+        type="int",
+        description="Number of assets added",
+        example="5",
+    ),
+    TemplateVariableDefinition(
+        name="removed_count",
+        type="int",
+        description="Number of assets removed",
+        example="2",
+    ),
+    TemplateVariableDefinition(
+        name="assets",
+        type="list",
+        description="List of added media assets with details",
+        example="[{id, type, filename, ...}]",
+    ),
+    TemplateVariableDefinition(
+        name="old_name",
+        type="string",
+        description="Previous collection name (for rename events)",
+        example="Old Album Name",
+    ),
+    TemplateVariableDefinition(
+        name="new_name",
+        type="string",
+        description="New collection name (for rename events)",
+        example="New Album Name",
+    ),
+]
+
+
+@dataclass
+class VariableRegistry:
+    """Registry of template variables, organized by provider type.
+
+    Maintains base variables (available to all providers) and
+    provider-specific variables. Queries return the union of both.
+    """
+
+    _provider_vars: dict[ServiceProviderType, list[TemplateVariableDefinition]] = field(
+        default_factory=dict
+    )
+
+    def register_provider_variables(
+        self,
+        provider_type: ServiceProviderType,
+        variables: list[TemplateVariableDefinition],
+    ) -> None:
+        """Register variables for a specific provider type."""
+        self._provider_vars[provider_type] = variables
+
+    def get_variables(
+        self, provider_type: ServiceProviderType
+    ) -> list[TemplateVariableDefinition]:
+        """Get all variables available for a provider type (base + provider-specific)."""
+        provider_vars = self._provider_vars.get(provider_type, [])
+        return BASE_VARIABLES + provider_vars
+
+    def get_base_variables(self) -> list[TemplateVariableDefinition]:
+        """Get base variables available to all providers."""
+        return list(BASE_VARIABLES)
+
+    def get_provider_specific_variables(
+        self, provider_type: ServiceProviderType
+    ) -> list[TemplateVariableDefinition]:
+        """Get only the provider-specific variables (excluding base)."""
+        return list(self._provider_vars.get(provider_type, []))
+
+    def get_variable_names(self, provider_type: ServiceProviderType) -> set[str]:
+        """Get the set of all variable names available for a provider type."""
+        return {v.name for v in self.get_variables(provider_type)}
+
+
+# Global registry instance
+registry = VariableRegistry()