Surface cross-source duplicates on artist + album views with resolve actions

Summary

• Artist detail page and the main Albums list now recognize when the same release exists in both the Sort and Sorted directories at different qualities, and show a contextual badge on each row (Upgrade available / Upgrade target / Lower quality / Already sorted).
• Both right-hand panels (AlbumOrganizePanel, SortedAlbumPanel) gain a side-by-side compare block with two end-to-end actions: Replace Sorted with Unsorted (existing executeSort + overwriteExisting=true) and Delete Unsorted copy (existing DELETE /api/albums/[id]).
• A new duplicate_group column on sorted_albums plus the existing one on albums lets the cross-table join run once per loader call. Scanner and sorter write the normalized key on every upsert; loaders fall back to in-memory computation when the column is still NULL so the UI works on day one.

Commit structure

1. d5c75b0 Persist duplicate-group key on sorted albums — schema + persistence foundation.
2. b7fd719 Surface cross-source matches in loaders and album endpoints.
3. 08aad2b Show duplicate badge on album rows and cards.
4. 15b6eb7 Compare versions and resolve duplicates from album panels.

Test plan

• pnpm run build passes (verified locally).
• After deploy, PRAGMA table_info(sorted_albums) shows duplicate_group.
• Open an artist with a known dupe (e.g. an album in Sorted as MP3 + Unsorted as FLAC): both rows show the expected colored pill.
• Click the Sorted row → compare block appears with Replace and Delete Unsorted buttons. Replace removes the old sorted folder and copies the unsorted version in; the pair disappears on reload.
• Repeat where the unsorted copy is lower quality and click Delete Unsorted copy — unsorted folder removed, sorted untouched.
• Albums list page (/) in grid and list view shows the pill on matching rows.
• Artist with no duplicates: no pills, no compare block.