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/objects.go

@@ -22,6 +22,7 @@ const (
 	RoleThumb    = "thumb"
 	RoleMedium   = "medium"
 	RolePreview  = "preview"
+	RolePlayback = "playback"
 )
 
 var (
@@ -285,8 +286,9 @@ func (s *Store) ClaimStalePending(ctx context.Context, olderThan time.Duration,
 
 // ClaimVariantJobs leases due variant jobs and returns their photo ids. The
 // lease pushes next_attempt_at forward so the same job is not picked up twice
-// while it is being processed.
-func (s *Store) ClaimVariantJobs(ctx context.Context, limit int, lease time.Duration) ([]string, error) {
+// while it is being processed. A non-empty kind restricts claims to that media
+// kind so long video work cannot starve image jobs.
+func (s *Store) ClaimVariantJobs(ctx context.Context, kind string, limit int, lease time.Duration) ([]string, error) {
 	if limit <= 0 {
 		limit = 10
 	}
@@ -300,12 +302,13 @@ func (s *Store) ClaimVariantJobs(ctx context.Context, limit int, lease time.Dura
 		`SELECT vj.photo_id FROM variant_jobs vj
 		 JOIN photos p ON p.id = vj.photo_id
 		 WHERE vj.next_attempt_at <= now()
+		   AND ($2 = '' OR vj.kind = $2)
 		 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 $1
-		 FOR UPDATE OF vj SKIP LOCKED`, limit)
+		 FOR UPDATE OF vj SKIP LOCKED`, limit, kind)
 	if err != nil {
 		return nil, err
 	}