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/migrations/000012_video_support.down.sql

@@ -0,0 +1,9 @@
+DELETE FROM photo_objects WHERE role = 'playback';
+
+ALTER TABLE photo_objects DROP CONSTRAINT IF EXISTS photo_objects_role_check;
+ALTER TABLE photo_objects ADD CONSTRAINT photo_objects_role_check
+    CHECK (role IN ('original', 'thumb', 'medium', 'preview'));
+
+ALTER TABLE variant_jobs DROP COLUMN IF EXISTS kind;
+ALTER TABLE photos DROP COLUMN IF EXISTS duration_ms;
+ALTER TABLE photos DROP COLUMN IF EXISTS kind;