Fix Phase 5 review issues: SSTI, FK violation, sync rebuild

Fixes 5 issues identified by code-reviewer agent:

1. (Critical) EventLog.tracker_id now nullable - use None instead
   of 0 when tracker name doesn't match, avoiding FK constraint
   violations on PostgreSQL
2. (Critical) Replace jinja2.Environment with SandboxedEnvironment
   in all 3 server template rendering locations to prevent SSTI
3. (Important) Rebuild sync_client in _async_update_listener when
   server URL/key options change, propagate to all coordinators
4. (Important) Validate partial server config - require both URL
   and API key or neither, with clear error message
5. (Important) Name fire-and-forget sync task for debugging

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

packages/server/src/immich_watcher_server/database/models.py

@@ -104,7 +104,7 @@ class EventLog(SQLModel, table=True):
     __tablename__ = "event_log"
 
     id: int | None = Field(default=None, primary_key=True)
-    tracker_id: int = Field(foreign_key="album_tracker.id")
+    tracker_id: int | None = Field(default=None, foreign_key="album_tracker.id")
     event_type: str
     album_id: str
     album_name: str