feat: add video streaming proxy and webhook cache invalidation

- Add /api/video/[id] route with Range header support for seeking
- Add /api/webhook route for Immich event-driven cache invalidation
- Add streamVideo() to ImmichClient with 206 Partial Content support
- Support album description overrides in gallery.yaml
- Add videoUrl() helper to lib/urls.ts
- Export cache.delete() and cache.clear() methods

lib/urls.ts

@@ -24,6 +24,13 @@ export function exifUrl(assetId: string): string {
   return `/api/exif/${encodeAssetId(assetId)}`;
 }
 
+/**
+ * Generate a public video proxy URL for an asset.
+ */
+export function videoUrl(assetId: string): string {
+  return `/api/video/${encodeAssetId(assetId)}`;
+}
+
 /**
  * Placeholder data derived from an asset's ThumbHash.
  */