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>

plans/phase-5-haos-server-sync.md

@@ -0,0 +1,49 @@
+# Phase 5: HAOS-Server Sync (Optional)
+
+**Status**: In progress
+**Parent**: [primary-plan.md](primary-plan.md)
+
+---
+
+## Goal
+
+Allow the HAOS integration to optionally connect to the standalone server, enabling:
+- Syncing tracker configurations from server to HA
+- Using server-managed templates for notifications
+- Centralized management via the web UI while HA provides entities/automations
+
+The connection is **additive** -- the integration works standalone without a server.
+
+---
+
+## Tasks
+
+### 1. Add server URL to HA config flow `[ ]`
+- Add optional `server_url` field to hub options flow
+- Add optional `server_api_key` field (JWT token or API key)
+- Validate connection on save
+
+### 2. Server sync service `[ ]`
+- New `sync.py` module in integration
+- `async_sync_from_server()`: Fetch tracker configs from server API
+- Map server trackers to HA album subentries
+- Store server connection state in hub data
+
+### 3. Server API endpoint for HA sync `[ ]`
+- `GET /api/sync/trackers` -- Returns tracker configs formatted for HA consumption
+- `GET /api/sync/templates/{id}/render` -- Render template with provided context
+- Auth via API key header (simpler than JWT for machine-to-machine)
+
+### 4. Bidirectional event reporting `[ ]`
+- HA integration reports detected events back to server
+- `POST /api/sync/events` -- HA pushes event data to server for logging
+- Server dashboard shows events from both standalone polling and HA
+
+---
+
+## Acceptance Criteria
+
+- [ ] Integration works identically without server URL configured
+- [ ] With server URL, tracker configs can be synced
+- [ ] Events detected by HA are visible in server dashboard
+- [ ] Server connection failure doesn't break HA integration