fix(config): wire DataDir from yaml/env through to playlists service

Slice 1 of M7 #352 added Server.DataDir but left it un-populated —
the playlist cover-collage writer would have used "" as the relative
path root in production, writing to the current working directory.

- Add Storage.DataDir to Config (yaml: storage.data_dir, env:
  SMARTMUSIC_STORAGE_DATA_DIR), defaulting to "./data".
- server.New takes the data dir as a parameter; main.go threads it.
- main.go MkdirAll's the directory at startup so the playlist cover
  writer doesn't fail on first use with a missing parent.
- config.example.yaml documents the new section.

Existing internal/server/server_test.go constructs Server directly
without server.New, so no test fixture changes needed for that file.

cmd/minstrel/main.go

@@ -97,9 +97,19 @@ func run() error {
 	lidarrReconciler := lidarrrequests.NewReconciler(pool, lidarrCfg, logger.With("component", "lidarr"))
 	go lidarrReconciler.Run(ctx)
 
+	// Ensure DataDir exists before any service tries to write into it.
+	// Best-effort: log + continue on failure; the playlist cover writer
+	// will surface a clearer error at first use.
+	if cfg.Storage.DataDir != "" {
+		if err := os.MkdirAll(cfg.Storage.DataDir, 0o755); err != nil {
+			logger.Warn("data_dir create failed; cached artifacts may not persist",
+				"path", cfg.Storage.DataDir, "err", err)
+		}
+	}
+
 	srv := server.New(logger, pool, scanner, subsonic.Config{
 		AllowPlaintextPassword: cfg.Subsonic.AllowPlaintextPassword,
-	}, cfg.Events, cfg.Recommendation)
+	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir)
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Address,
 		Handler:           srv.Router(),