feat: Collection › Unmatched lists the library albums no MusicBrainz album matched, each matched by hand from a search or a link, or left off (M502 #5579)

Like Lidarr's Unmapped Files. A local band's album, which MusicBrainz will
never have, is left off: never matched again and not counted. A re-tagged
album is matched again on the next listing. The Collection's verdict now
says when library albums need matching.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/api.go

@@ -253,6 +253,10 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 				// profiles that decide which of their releases count.
 				admin.Get("/wanted", h.handleListWants)
 				admin.Get("/wanted/artists", h.handleListWantArtists)
+				admin.Get("/wanted/unmatched", h.handleListUnmatched)
+				admin.Post("/wanted/unmatched/{id}/match", h.handleMatchUnmatched)
+				admin.Post("/wanted/unmatched/{id}/leave-off", h.handleLeaveUnmatchedOff)
+				admin.Post("/wanted/unmatched/{id}/put-back", h.handlePutUnmatchedBack)
 				admin.Delete("/wanted/{id}", h.handleRemoveWant)
 				// Searching and grabbing (M502 #5521).
 				admin.Get("/acquisition/settings", h.handleGetSearchSettings)