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>

internal/coverart/artist_enricher.go

@@ -144,15 +144,21 @@ func (e *Enricher) recordArtistArt(ctx context.Context, artistID pgtype.UUID, th
 // errored). The JSON tally written to scan_runs keeps the existing
 // processed/succeeded/failed shape for backwards compat with the
 // admin overview UI; the log line is the operator's diagnostic.
+// limit: 0 = stage disabled; >0 = bounded; <0 = unbounded (#388 —
+// no global cap; remote providers self-throttle per provider).
 func (e *Enricher) EnrichArtistBatch(ctx context.Context, limit int, dataDir string,
 	progressCb func(processed, succeeded, failed int)) (processed, succeeded, failed int, err error) {
-	if limit <= 0 {
+	if limit == 0 {
 		return 0, 0, 0, nil
 	}
+	queryLimit := int32(limit)
+	if limit < 0 {
+		queryLimit = 1<<31 - 1 // unbounded
+	}
 	q := dbq.New(e.pool)
 	rows, qerr := q.ListArtistsMissingArt(ctx, dbq.ListArtistsMissingArtParams{
 		ArtistArtSourcesVersion: e.settings.CurrentVersion(),
-		Limit:                   int32(limit),
+		Limit:                   queryLimit,
 	})
 	if qerr != nil {
 		return 0, 0, 0, fmt.Errorf("list missing artist art: %w", qerr)