Every scan (incremental / full / deep, both entry points) now steps the
enrichment workers aside to cut DB lock contention — same as music. Mirrors
music's contract exactly: pause ONLY workers that were running (a user's manual
pause is left alone), track which we paused, and resume just those in a finally
so success, cancel, and error all clean up. Auto-pause is transient
(persist=False) so it never leaks into the saved <service>_paused flag.
Hooks are injected by get_video_scanner; the scanner is inert without them
(tests build it directly, no engine spun up). Isolated to the video side.
Pause/resume now write <service>_paused to video_settings, and the engine
restores each worker's saved pause when it's (re)built — mirrors music's
<service>_enrichment_paused boot flag. Isolated to video.db; music untouched.
