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
alexei.dolgolyov
307871cae5