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>

custom_components/immich_album_watcher/translations/en.json

@@ -81,7 +81,8 @@
             "invalid_auth": "Invalid API key",
             "no_albums": "No albums found on the server",
             "unknown": "Unexpected error occurred",
-            "server_connect_failed": "Failed to connect to Immich Watcher server"
+            "server_connect_failed": "Failed to connect to Immich Watcher server",
+            "server_partial_config": "Both server URL and API key are required (or leave both empty to disable sync)"
         },
         "abort": {
             "already_configured": "This Immich server is already configured"