Add standalone FastAPI server backend (Phase 3)

Build a complete standalone web server for Immich album change
notifications, independent of Home Assistant. Uses the shared
core library from Phase 1.

Server features:
- FastAPI with async SQLite (SQLModel + aiosqlite)
- Multi-user auth with JWT (admin/user roles, setup wizard)
- CRUD APIs: Immich servers, album trackers, message templates,
  notification targets (Telegram + webhook), user management
- APScheduler background polling per tracker
- Jinja2 template rendering with live preview
- Album browser proxied from Immich API
- Event logging and dashboard status endpoint
- Docker deployment (single container, SQLite in volume)

39 API routes, 14 integration tests passing.

Also adds Phase 6 (Claude AI Telegram bot enhancement) to the
primary plan as an optional future phase.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/server/src/immich_watcher_server/webhook/__init__.py

@@ -0,0 +1 @@
+"""Webhook notification provider."""

packages/server/src/immich_watcher_server/webhook/client.py

@@ -0,0 +1,41 @@
+"""Generic webhook notification client."""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+import aiohttp
+
+_LOGGER = logging.getLogger(__name__)
+
+
+class WebhookClient:
+    """Send JSON payloads to a webhook URL."""
+
+    def __init__(self, session: aiohttp.ClientSession, url: str, headers: dict[str, str] | None = None) -> None:
+        self._session = session
+        self._url = url
+        self._headers = headers or {}
+
+    async def send(self, payload: dict[str, Any]) -> dict[str, Any]:
+        """POST JSON payload to the webhook URL.
+
+        Returns:
+            Dict with success status and optional error.
+        """
+        try:
+            async with self._session.post(
+                self._url,
+                json=payload,
+                headers={"Content-Type": "application/json", **self._headers},
+            ) as response:
+                if 200 <= response.status < 300:
+                    _LOGGER.debug("Webhook sent successfully to %s (HTTP %d)", self._url, response.status)
+                    return {"success": True, "status_code": response.status}
+                body = await response.text()
+                _LOGGER.error("Webhook failed: HTTP %d - %s", response.status, body[:200])
+                return {"success": False, "error": f"HTTP {response.status}", "body": body[:200]}
+        except aiohttp.ClientError as err:
+            _LOGGER.error("Webhook request failed: %s", err)
+            return {"success": False, "error": str(err)}