feat(video): add video upload with remux/transcode playback

Add video as a media kind in photos: streaming upload with per-kind
limits, ffprobe metadata, poster frame reusing the image variant
pipeline, and a playback object (remux when web-compatible, else
h264/yuv420p/aac transcode). Video jobs run on a separate pool with a
long lease. Frontend gains a video viewer, grid badge/duration and
video upload.

backend/internal/storage/fs.go

@@ -122,6 +122,22 @@ func (f *FS) Get(_ context.Context, key string) ([]byte, error) {
 	return data, nil
 }
 
+// Open streams a blob from disk for processing without buffering it fully.
+func (f *FS) Open(_ context.Context, key string) (io.ReadCloser, error) {
+	p, err := f.objectPath(key)
+	if err != nil {
+		return nil, err
+	}
+	file, err := os.Open(p)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil, ErrObjectNotFound
+		}
+		return nil, err
+	}
+	return file, nil
+}
+
 func (f *FS) Delete(_ context.Context, keys ...string) error {
 	var errs []error
 	for _, key := range keys {