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_test.go

@@ -51,6 +51,29 @@ func TestFSRoundTrip(t *testing.T) {
 	}
 }
 
+func TestFSOpen(t *testing.T) {
+	ctx := context.Background()
+	fs := newTestFS(t)
+	key := "objects/abc/original.mp4"
+	data := []byte("streamed video bytes")
+
+	if err := fs.Put(ctx, key, bytes.NewReader(data), int64(len(data)), "video/mp4"); err != nil {
+		t.Fatalf("put: %v", err)
+	}
+	rc, err := fs.Open(ctx, key)
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	got, err := io.ReadAll(rc)
+	rc.Close()
+	if err != nil || !bytes.Equal(got, data) {
+		t.Fatalf("open read = %q err=%v, want %q", got, err, data)
+	}
+	if _, err := fs.Open(ctx, "objects/missing/original.mp4"); !errors.Is(err, ErrObjectNotFound) {
+		t.Fatalf("open missing err = %v, want ErrObjectNotFound", err)
+	}
+}
+
 func TestFSPathTraversalRejected(t *testing.T) {
 	ctx := context.Background()
 	fs := newTestFS(t)