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/api/admin_covers_test.go

@@ -53,7 +53,6 @@ func testHandlersWithCovers(t *testing.T) (*handlers, *coverart.Enricher) {
 	}
 	enricher := coverart.NewEnricher(pool, slog.Default(), settings)
 	h.coverart = enricher
-	h.coverArtBackfillCap = 100
 	return h, enricher
 }
 
@@ -97,7 +96,7 @@ func TestAdminAlbumRefetchCover_AdminOK(t *testing.T) {
 	}
 }
 
-func TestAdminBulkRefetchCovers_AdminReturnsQueuedCount(t *testing.T) {
+func TestAdminBulkRefetchCovers_AdminStartsRefetch(t *testing.T) {
 	if os.Getenv("MINSTREL_TEST_DATABASE_URL") == "" {
 		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
 	}
@@ -115,8 +114,8 @@ func TestAdminBulkRefetchCovers_AdminReturnsQueuedCount(t *testing.T) {
 	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
 		t.Fatalf("decode: %v", err)
 	}
-	if resp.Queued != h.coverArtBackfillCap {
-		t.Errorf("queued = %d, want %d (cap)", resp.Queued, h.coverArtBackfillCap)
+	if !resp.Started {
+		t.Errorf("started = false, want true (bulk refetch should acknowledge)")
 	}
 }