feat(webhook): implement Phase 1 webhook receiver for Sonarr and Radarr

- 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

.env.sample

@@ -19,6 +19,16 @@ LOG_LEVEL=info
 # Generate with: openssl rand -hex 32
 COOKIE_SECRET=your-cookie-secret-here
 
+# =============================================================================
+# WEBHOOK SETTINGS
+# =============================================================================
+
+# Secret for validating incoming webhooks from Sonarr and Radarr
+# Required for webhook endpoints to accept requests
+# Sonarr/Radarr must send this secret in the X-Sofarr-Webhook-Secret header
+# Generate with: openssl rand -hex 32
+SOFARR_WEBHOOK_SECRET=your-webhook-secret-here
+
 # =============================================================================
 # TLS / HTTPS
 # =============================================================================