feat: add visual admin panel with page builder and settings editor

- Add /admin route with password-protected visual page builder
- Page Builder: manage hero images, standalone albums, subpages, and sections
- Album Picker: browse all shared Immich albums with search and filtering
- Settings Editor: theme, grid, footer, legal, SEO configuration
- YAML service with atomic writes and automatic backups (content/.backups/)
- HMAC-signed admin sessions (24h expiry, HttpOnly cookies)
- API endpoints: /api/admin/{auth,gallery,settings,albums,reload}
- Enable with ADMIN_PASSWORD env var; disabled by default
- invalidateConfigCache() export for hot-reload after saves

.env.local.example

@@ -30,6 +30,17 @@ CACHE_TTL=300
 # If not provided, IMMICH_API_KEY will be used as a fallback (not recommended).
 # AUTH_SECRET=your-super-long-random-secret-here
 
+# Webhook secret for cache invalidation (optional, string).
+# Set this to a long random string and configure Immich to send webhooks to
+# POST /api/webhook with an x-immich-signature header (HMAC-SHA256 of the body).
+# If not set, the webhook endpoint returns 501 and is effectively disabled.
+# WEBHOOK_SECRET=your-long-random-webhook-secret
+
+# ── Admin Panel ──────────────────────────────────────────────────
+# Set a password to enable the visual admin panel at /admin.
+# Leave unset or empty to disable the admin UI entirely.
+# ADMIN_PASSWORD=your-secure-admin-password
+
 # ── Album configuration ──────────────────────────────────────────
 # Albums, subpages, hero image, and passwords are defined in content/gallery.yaml
 # See content/gallery.yaml.example for the full reference.