feat: admin Suspect sources report, for files named like video rips (#5410)

Humanz turned out to be YouTube rips: "(Official Video)", "Visualizer", a
reaction video filed as a song, junk disc numbers (#5401). The library holds
about 200 more files named the same way. This report lists them, grouped by
folder like Missing files, with the markers each file name carries.

- GET /api/admin/library/suspect-sources: one marker list in Go builds both
  the Postgres ~* filter and each row's labels, so they cannot drift.
  Basename only; missing files are left out.
- Markers calibrated on the live library: "live in/at" dropped (real live
  albums), "reaction" narrowed (it caught "Chain Reaction").
- Admin tab "Suspect sources", read-only, loads as you scroll; a folder split
  across pages is joined back into one.

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

internal/api/api.go

@@ -252,6 +252,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 				// and because the destructive /tracks/{id} route above must
 				// not be mistaken for it (#2527).
 				admin.Get("/library/missing", h.handleListMissingTracks)
+				admin.Get("/library/suspect-sources", h.handleListSuspectSources)
 
 				admin.Get("/library/coverage", h.handleGetLibraryCoverage)
 				admin.Get("/library/fingerprints", h.handleGetFingerprintCoverage)