feat: Google Photos provider backend + API hardening

- Add Google Photos provider: client, models, change detector, capabilities
- Add notification templates (en/ru) for all GP event slots
- Add command templates (en/ru) for GP bot commands
- Register GP in slot/command loaders, capabilities, and seeds
- Harden provider API: validate OAuth credentials on create/update
- Add internal URL rewriting for asset fetches (LAN optimization)
- Fix template renderer to handle missing variables gracefully
- Improve webhook command routing for multi-provider support
- Add provider health check endpoint and watcher improvements

packages/server/src/notify_bridge_server/api/template_configs.py

@@ -130,7 +130,9 @@ async def list_configs(
 
 
 @router.get("/variables")
-async def get_template_variables():
+async def get_template_variables(
+    user: User = Depends(get_current_user),
+):
     """Get template variable reference grouped by slot.
 
     Returns a dict keyed by template slot name, each containing: