Add HAOS-Server sync for optional centralized management (Phase 5)

Enable the HAOS integration to optionally connect to the standalone
Immich Watcher server for config sync and event reporting.

Server-side:
- New /api/sync/* endpoints: GET trackers, POST template render,
  POST event report
- API key auth via X-API-Key header (accepts JWT access tokens)

Integration-side:
- New sync.py: ServerSyncClient with graceful error handling
  (all methods return defaults on connection failure)
- Options flow: optional server_url and server_api_key fields
  with connection validation
- Coordinator: fire-and-forget event reporting to server when
  album changes are detected
- Translations: en.json and ru.json updated with new fields

The connection is fully additive -- the integration works identically
without a server URL configured. Server failures never break HA.

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

custom_components/immich_album_watcher/const.py

@@ -66,6 +66,8 @@ CONF_ALBUM_NAME: Final = "album_name"
 CONF_SCAN_INTERVAL: Final = "scan_interval"
 CONF_TELEGRAM_BOT_TOKEN: Final = "telegram_bot_token"
 CONF_TELEGRAM_CACHE_TTL: Final = "telegram_cache_ttl"
+CONF_SERVER_URL: Final = "server_url"
+CONF_SERVER_API_KEY: Final = "server_api_key"
 
 # Subentry type
 SUBENTRY_TYPE_ALBUM: Final = "album"