feat(server/m7-cover-sources): album enricher uses provider chain

Rewires EnrichAlbum to iterate EnabledAlbumProviders from the
SettingsService instead of a single hardcoded MBCAA fetcher. Sidecar
layer is unchanged (always tried first). Per-row eligibility check
honors cover_art_sources_version: terminal 'found' values skip;
'none' flips to NULL via ClearAlbumCoverNone and proceeds (the DB-
level ListAlbumsMissingCover query guards the version check for batch
callers; RetryAlbum clears via ClearAlbumCover first); NULL is eligible.

The provider chain uses an inline allWere404 accumulator to settle
the row to 'none' only when every provider returned ErrNotFound.
Any provider returning ErrTransient leaves the row NULL for next-
pass retry — even if other providers returned ErrNotFound — since
the transient call's MBID might succeed on a future attempt.

Boot wiring (cmd/minstrel/main.go) replaces the
NewFetcher+NewEnricher(fetcher, mbcaaOn) shape with
NewMBCAAProviderFromConfig (swaps in production User-Agent on the
registered provider) + NewSettingsService + NewEnricher(settings).
The old cfg.Library.CoverArtFromMBCAA flag is no-op'd; DB-backed
settings replace it. Field stays in the config struct for backward
compat with deployed config files.

Consolidates fetcher.go's HTTP logic into provider_mbcaa.go (no more
wrapper indirection): mbcaaProvider now holds cfg/mu/lastCall/client
directly. Deletes fetcher.go and fetcher_test.go. ErrNotFound and
ErrTransient move into provider.go alongside the other sentinels.

Updates admin_covers_test.go and provider_mbcaa_test.go to the new
constructor shapes. Adds multi-provider-chain, transient-leaves-null,
and stale-none-flips-and-retries test cases in enricher_test.go.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -77,11 +77,20 @@ func run() error {
 	if contact == "" {
 		contact = "https://git.fabledsword.com/bvandeusen/minstrel"
 	}
-	coverFetcher := coverart.NewFetcher(coverart.FetcherConfig{
+	// Swap in the production User-Agent + MinPeriod on the registered
+	// MBCAA provider. init() registered it with default config; this
+	// updates the running instance.
+	coverart.NewMBCAAProviderFromConfig(coverart.FetcherConfig{
 		UserAgent: fmt.Sprintf("Minstrel/dev (%s)", contact),
 		MinPeriod: time.Second,
 	})
-	coverEnricher := coverart.NewEnricher(pool, logger.With("component", "coverart"), coverFetcher, cfg.Library.CoverArtFromMBCAA)
+
+	coverSettings, err := coverart.NewSettingsService(ctx, pool, logger.With("component", "coverart"))
+	if err != nil {
+		logger.Error("coverart settings service init failed", "err", err)
+		os.Exit(1)
+	}
+	coverEnricher := coverart.NewEnricher(pool, logger.With("component", "coverart"), coverSettings)
 
 	// One unified scan chain: library walk → MBID backfill → cover enrich.
 	// Boot-time scan and manual-trigger scans share this path; results land