Unlike the cleanup twins, backup can't share the music handler — it's a different
DB file. Extract the music backup body into _backup_db_at(db_path, ...) (music
behaviour byte-identical, now a thin wrapper over DATABASE_PATH) and add
auto_backup_video_database pointing at VIDEO_DATABASE_PATH (video_library.db).
New video_backup_database action (scope='video' block + registry), owned_by='video'
system automation on the music cadence (every 3 days).
Tests: a REAL backup behaviour test — music backup lands next to music_library.db,
video backup next to video_library.db, no cross-contamination (this is the whole
reason it can't be shared); scope isolation; single video-owned seed; own handler.
Existing music maintenance tests (22) still green — refactor is non-regressing.
EXPECTED_ACTION_NAMES updated.
3937cb4cb8