Introduce a new Retag tool to track and re-tag previously downloaded albums/singles. Changes include:
- Database: add migration hook and create retag_groups and retag_tracks tables, indexes, and many helper methods (add/find/update/delete groups & tracks, stats, trimming).
- Backend (web_server): capture completed album/single downloads into the retag tables, implement retag execution logic (_execute_retag) to fetch album metadata, match tracks, update tags, move files, download cover art, and update DB. Add thread-safe globals, executor, and REST endpoints for stats, listing groups, group tracks, album search, execute, status, and delete.
- Frontend (webui): add Retag Tool card, modals, search UI, JS to list groups/tracks, search albums, start retag operations, poll status, and update UI; include help content. Add CSS for modals and components.
The migration is invoked during DB init to ensure existing installations create the new tables. The tool caps stored groups (default 100) and avoids duplicate track entries.
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