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/sqlite_objects.go

@@ -226,8 +226,9 @@ func (s *SQLiteStore) ClaimStalePending(ctx context.Context, olderThan time.Dura
 	return objs, nil
 }
 
-// ClaimVariantJobs leases due variant jobs and returns their photo ids.
-func (s *SQLiteStore) ClaimVariantJobs(ctx context.Context, limit int, lease time.Duration) ([]string, error) {
+// ClaimVariantJobs leases due variant jobs and returns their photo ids. A
+// non-empty kind restricts claims to that media kind.
+func (s *SQLiteStore) ClaimVariantJobs(ctx context.Context, kind string, limit int, lease time.Duration) ([]string, error) {
 	if limit <= 0 {
 		limit = 10
 	}
@@ -243,11 +244,12 @@ func (s *SQLiteStore) ClaimVariantJobs(ctx context.Context, limit int, lease tim
 		`SELECT vj.photo_id FROM variant_jobs vj
 		 JOIN photos p ON p.id = vj.photo_id
 		 WHERE vj.next_attempt_at <= $1
+		   AND ($3 = '' OR vj.kind = $3)
 		 ORDER BY (vj.attempts > 0) ASC,
 		          CASE WHEN vj.attempts = 0 THEN p.created_at END DESC NULLS LAST,
 		          vj.next_attempt_at ASC,
 		          vj.photo_id DESC
-		 LIMIT $2`, now, limit)
+		 LIMIT $2`, now, limit, kind)
 	if err != nil {
 		return nil, err
 	}