feat(server/m7-cover-sources): artist art enricher (thumb + fanart)

Parallels EnrichAlbum but with no sidecar layer (artists have no
per-artist directory in a music library). Iterates
EnabledArtistProviders, writes thumb.jpg + fanart.jpg to
<data_dir>/artist-art/<artist_id>/, stamps artist_art_source +
artist_art_sources_version atomically.

Atomicity: provider returns whichever images it has — partial
success (thumb-only or fanart-only) is persisted with whichever
paths landed, source still stamped. ErrNotFound from every enabled
provider settles the row to 'none' with the current version stamp.
Any ErrTransient leaves the row NULL for next-pass retry.

CleanupArtistArt removes <data_dir>/artist-art/<artist_id>/ on
artist delete; idempotent on missing dir. Hooked into the artist-
delete cascade in tracks/service.go after the transaction commits;
non-fatal on failure (logs but doesn't fail the delete).

EnrichArtistBatch drains rows from ListArtistsMissingArt; the
orchestrator's 4th stage (added in T10) calls this. Tests cover
the eligibility table, atomicity (thumb-only / fanart-only),
all-404-settles, ErrTransient-leaves-NULL, filesystem layout, and
CleanupArtistArt idempotency.

tracks.Service gains a dataDir field; tracks.NewService takes it
as a new parameter. All three call sites updated (server/server.go,
api/auth_test.go, api/admin_tracks_test.go). The dataDir flows from
cfg.Storage.DataDir → server.New → s.DataDir → tracks.NewService
without any change to cmd/minstrel/main.go.

internal/server/server.go

@@ -109,7 +109,7 @@ func (s *Server) Router() http.Handler {
 		}
 		lidarrReqs := lidarrrequests.NewService(s.Pool, lidarrCfg, lidarrClientFn, nil)
 		lidarrQuar := lidarrquarantine.NewService(s.Pool, lidarrCfg, lidarrClientFn)
-		tracksSvc := tracks.NewService(s.Pool, s.Logger, lidarrUnmonitorAdapter{fn: lidarrClientFn})
+		tracksSvc := tracks.NewService(s.Pool, s.Logger, lidarrUnmonitorAdapter{fn: lidarrClientFn}, s.DataDir)
 		playlistsSvc := playlists.NewService(s.Pool, s.Logger, s.DataDir)
 		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg, lidarrCfg, lidarrReqs, lidarrQuar, tracksSvc, playlistsSvc, s.CoverEnricher, s.CoverArtBackfillCap, s.LibraryScanner, s.ScanCfg, s.DataDir)
 		// /api/admin/scan is the only admin route owned by the server package