feat(coverart): #388 remove global cover-art backfill cap

Operator feedback (2026-05-09): the 500-album cap meant cover art rolled
in over many nightly runs even when local sources (sidecar/embedded)
could finish in minutes. Remove the global cap; rely on the existing
per-provider HTTP throttle (coverart httpClient MinInterval) so local
art is disk-speed and remote providers stay TOS-friendly.

- enricher.go / artist_enricher.go: EnrichBatch, EnrichArtistBatch,
  EnrichRetryMissing now treat limit<0 as unbounded (0 still = stage
  disabled). The cap was a SQL LIMIT; unbounded uses max int32.
- main.go: RunScanConfig EnrichCap/ArtistEnrichCap = -1 (unbounded).
- Drop LibraryConfig.CoverArtBackfillCap + the
  MINSTREL_LIBRARY_COVERART_BACKFILL_CAP env var.
- Drop the now-dead coverBackfillCap param threaded through
  server.New + api.Mount + the handlers struct.
- Admin bulk refetch (/api/admin/covers/refetch-missing) now drains
  unbounded; response {queued:int} → {started:bool} (the count is
  unknowable synchronously for a fire-and-forget drain). Web copy +
  client type + Go/web tests updated to match.

No doc refs existed (config.example.yaml / docker-compose / README
never documented the env var).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -105,8 +105,8 @@ func run() error {
 				logger.With("component", "scan_run"),
 				library.RunScanConfig{
 					BackfillCap:     5000,
-					EnrichCap:       cfg.Library.CoverArtBackfillCap,
-					ArtistEnrichCap: cfg.Library.CoverArtBackfillCap,
+					EnrichCap:       -1, // #388: no global cap; per-provider rate limits
+					ArtistEnrichCap: -1,
 					DataDir:         cfg.Storage.DataDir,
 				},
 			); err != nil {
@@ -122,8 +122,8 @@ func run() error {
 				logger.With("component", "scan_run"),
 				library.RunScanConfig{
 					BackfillCap:     5000,
-					EnrichCap:       cfg.Library.CoverArtBackfillCap,
-					ArtistEnrichCap: cfg.Library.CoverArtBackfillCap,
+					EnrichCap:       -1, // #388: no global cap; per-provider rate limits
+					ArtistEnrichCap: -1,
 					DataDir:         cfg.Storage.DataDir,
 				},
 			); err != nil {
@@ -194,8 +194,8 @@ func run() error {
 
 	scanCfg := library.RunScanConfig{
 		BackfillCap:     5000,
-		EnrichCap:       cfg.Library.CoverArtBackfillCap,
-		ArtistEnrichCap: cfg.Library.CoverArtBackfillCap,
+		EnrichCap:       -1, // #388: no global cap; per-provider rate limits
+		ArtistEnrichCap: -1,
 		DataDir:         cfg.Storage.DataDir,
 	}
 	scheduler := library.NewScheduler(pool, logger.With("component", "scheduler"),
@@ -203,7 +203,7 @@ func run() error {
 	scheduler.Start(ctx)
 	srv := server.New(logger, pool, scanner, subsonic.Config{
 		AllowPlaintextPassword: cfg.Subsonic.AllowPlaintextPassword,
-	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, cfg.Library.CoverArtBackfillCap, coverSettings, scanner, scanCfg, scheduler)
+	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, coverSettings, scanner, scanCfg, scheduler)
 	srv.Bus = bus
 	srv.PlaylistScheduler = playlistScheduler
 	httpServer := &http.Server{