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/cache.ts

@@ -44,6 +44,10 @@ class MemoryCache {
     });
   }
 
+  delete(key: string): void {
+    this.store.delete(key);
+  }
+
   clear(): void {
     this.store.clear();
   }