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/sqlite/000004_video_support.up.sql

@@ -0,0 +1,24 @@
+-- Video support for SQLite: media kind and duration on photos, the playback
+-- object role (photo_objects is rebuilt because SQLite cannot alter a CHECK
+-- constraint) and a job kind distinguishing image work from video transcoding.
+
+ALTER TABLE photos ADD COLUMN kind TEXT NOT NULL DEFAULT 'image';
+ALTER TABLE photos ADD COLUMN duration_ms INTEGER;
+
+CREATE TABLE photo_objects_new (
+    photo_id  TEXT NOT NULL REFERENCES photos(id) ON DELETE CASCADE,
+    object_id TEXT NOT NULL REFERENCES objects(id) ON DELETE RESTRICT,
+    role      TEXT NOT NULL CHECK (role IN ('original', 'thumb', 'medium', 'preview', 'playback')),
+    PRIMARY KEY (photo_id, role)
+);
+
+INSERT INTO photo_objects_new (photo_id, object_id, role)
+SELECT photo_id, object_id, role FROM photo_objects;
+
+DROP TABLE photo_objects;
+
+ALTER TABLE photo_objects_new RENAME TO photo_objects;
+
+CREATE INDEX IF NOT EXISTS photo_objects_object_idx ON photo_objects (object_id);
+
+ALTER TABLE variant_jobs ADD COLUMN kind TEXT NOT NULL DEFAULT 'image';