refactor: provider-agnostic bot command system + Gitea commands

Refactored the monolithic command handler (707 lines) into a pluggable
provider-handler architecture:

- Abstract ProviderCommandHandler interface (base.py)
- Handler dispatch registry routes commands by provider type
- Extracted all Immich logic into ImmichCommandHandler
- New GiteaCommandHandler with /status, /repos, /issues, /prs, /commits
- Multi-provider routing: groups context by provider type, finds handler
- handler.py reduced to ~280 line thin orchestrator

Gitea commands:
- Extended GiteaClient with get_repo_issues, get_repo_pulls, get_repo_commits
- 30 Jinja2 command templates (15 EN + 15 RU)
- Gitea capabilities updated with 6 commands + 15 command_slots
- Default command config + command template config seeded on startup
- Rate limiting: Gitea API commands share "api" category (15s cooldown)

Also:
- Command configs API accepts "gitea" provider type
- System command configs (user_id=0) visible to all users
- Webhook URL shown on Gitea provider card and edit form
- Scan interval hidden for webhook-based providers

packages/server/src/notify_bridge_server/commands/base.py

@@ -0,0 +1,66 @@
+"""Abstract provider command handler interface."""
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import Any
+
+from ..database.models import CommandTracker, CommandConfig, ServiceProvider, TelegramBot
+
+
+class ProviderCommandHandler(ABC):
+    """Base class for provider-specific bot command handlers.
+
+    Each provider (Immich, Gitea, etc.) implements this interface to handle
+    its own set of commands. The dispatch layer routes commands to the
+    correct handler based on the provider type.
+    """
+
+    provider_type: str
+
+    @abstractmethod
+    def get_provider_commands(self) -> set[str]:
+        """Return the set of command names this handler owns.
+
+        These are provider-specific commands (e.g., 'albums' for Immich,
+        'repos' for Gitea). Universal commands like 'help' and 'start'
+        are handled by the main dispatcher.
+        """
+
+    @abstractmethod
+    async def handle(
+        self,
+        cmd: str,
+        args: str,
+        count: int,
+        locale: str,
+        response_mode: str,
+        providers_map: dict[int, ServiceProvider],
+        cmd_templates: dict[str, dict[str, str]],
+        bot: TelegramBot,
+        ctx_tuples: list[tuple[CommandTracker, CommandConfig, ServiceProvider]],
+    ) -> str | list[dict[str, Any]] | None:
+        """Handle a provider-specific command.
+
+        Args:
+            cmd: The command name (without '/').
+            args: Arguments after the command.
+            count: Number of results to return.
+            locale: User's locale ('en', 'ru').
+            response_mode: 'media' or 'text'.
+            providers_map: Provider instances keyed by ID.
+            cmd_templates: Template slots {slot_name: {locale: template}}.
+            bot: The Telegram bot instance.
+            ctx_tuples: Command context tuples for this provider type.
+
+        Returns:
+            Text response, media list, or None if unhandled.
+        """
+
+    def get_rate_categories(self) -> dict[str, str]:
+        """Return rate limit category mapping for this provider's commands.
+
+        Keys are command names, values are category strings.
+        Commands not listed default to 'default' category.
+        """
+        return {}