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/photos/media.go

@@ -0,0 +1,16 @@
+package photos
+
+// Media kinds. A photo row may describe either a still image or a video.
+const (
+	KindImage = "image"
+	KindVideo = "video"
+)
+
+// mediaKind normalizes a photo kind, defaulting to image. It is used before
+// persisting so the column and the variant job always carry a valid value.
+func mediaKind(kind string) string {
+	if kind == KindVideo {
+		return KindVideo
+	}
+	return KindImage
+}