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/library/scanrun.go

@@ -49,9 +49,11 @@ type ArtistArtEnrichStageTallies struct {
 	Failed    int `json:"failed"`
 }
 
-// RunScanConfig assembles the knobs for the orchestrator. BackfillCap and
-// EnrichCap mirror the existing boot-time caps (5000 / coverArtBackfillCap)
-// so manual triggers don't accidentally drain the whole library.
+// RunScanConfig assembles the knobs for the orchestrator. BackfillCap is
+// the album/artist-MBID-backfill batch cap (5000); EnrichCap /
+// ArtistEnrichCap gate cover/artist-art enrichment (0 = off, <0 =
+// unbounded — #388 removed the global cover-art cap; remote providers
+// self-throttle per provider).
 type RunScanConfig struct {
 	BackfillCap     int
 	EnrichCap       int