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>

web/src/lib/api/admin.covers.test.ts

@@ -25,9 +25,9 @@ describe('admin covers API', () => {
   });
 
   it('refetchMissingCovers POSTs to the bulk endpoint', async () => {
-    (api.post as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({ queued: 7 });
+    (api.post as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({ started: true });
     const got = await refetchMissingCovers();
     expect(api.post).toHaveBeenCalledWith('/api/admin/covers/refetch-missing', {});
-    expect(got.queued).toBe(7);
+    expect(got.started).toBe(true);
   });
 });

web/src/lib/api/admin.ts

@@ -187,7 +187,7 @@ export async function refetchAlbumCover(albumId: string): Promise<RefetchAlbumCo
 }
 
 export type RefetchMissingResponse = {
-  queued: number;
+  started: boolean;
 };
 
 export async function refetchMissingCovers(): Promise<RefetchMissingResponse> {