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.go

@@ -48,19 +48,20 @@ func (h *handlers) handleAdminAlbumRefetchCover(w http.ResponseWriter, r *http.R
 }
 
 type adminBulkRefetchResp struct {
-	Queued int `json:"queued"`
+	Started bool `json:"started"`
 }
 
 // handleAdminBulkRefetchCovers implements POST /api/admin/covers/refetch-missing.
-// Returns immediately; actual drain runs in a background goroutine bounded by
-// the configured backfill cap.
+// Returns immediately; the drain runs UNBOUNDED in a background goroutine
+// (#388 — no global cap; remote providers self-throttle per provider, local
+// sources run at disk speed). The count is unknowable synchronously, so the
+// response just acknowledges the drain started.
 func (h *handlers) handleAdminBulkRefetchCovers(w http.ResponseWriter, _ *http.Request) {
-	cap := h.coverArtBackfillCap
 	go func() {
 		bgCtx := context.Background()
-		if _, err := h.coverart.EnrichRetryMissing(bgCtx, cap); err != nil {
+		if _, err := h.coverart.EnrichRetryMissing(bgCtx, -1); err != nil {
 			h.logger.Warn("admin: bulk cover refetch failed", "err", err)
 		}
 	}()
-	writeJSON(w, http.StatusOK, adminBulkRefetchResp{Queued: cap})
+	writeJSON(w, http.StatusOK, adminBulkRefetchResp{Started: true})
 }