- Added POST /api/webhook/sonarr and POST /api/webhook/radarr endpoints
- Implemented webhook secret validation via SOFARR_WEBHOOK_SECRET environment variable
- Added logging for all incoming webhook events using existing logToFile utility
- Returns HTTP 200 immediately to prevent webhook retries
- Mounted webhook routes before CSRF middleware (called by external services)
- Non-breaking: no changes to polling, caching, SSE, or any existing behavior
- Lays groundwork for Phase 2 (cache + SSE integration) without implementing it yet
