fix: instant video thumbnails on tab switch + throttle decodes

Backend:
- 204 response: add Cache-Control: no-store so browser never caches
  'no thumbnail yet' — next GET after PUT upload gets the WebP

Frontend:
- _videoThumbCache (Map): persists fileId → URL across re-renders,
  so switching tabs reuses cached URLs instantly (no re-decode)
- Render: videos with cached URL skip the 204/error/decode cycle
- After PUT succeeds: swap blob URL → server ?v=1 URL so blob is GC'd
- Concurrency throttle: max 3 simultaneous video decodes to avoid
  overwhelming network + CPU when gallery has many videos
- Decode queue: pending videos processed as slots free up

src/interfaces/api/handlers/file_handler.rs

@@ -349,7 +349,12 @@ impl FileHandler {
                     .unwrap()
                     .into_response();
             }
-            return StatusCode::NO_CONTENT.into_response();
+            return Response::builder()
+                .status(StatusCode::NO_CONTENT)
+                .header(header::CACHE_CONTROL, "no-store")
+                .body(Body::empty())
+                .unwrap()
+                .into_response();
         }
 
         // Resolve the physical blob path (content-addressable storage)