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(),

internal/config/config.go

@@ -11,6 +11,7 @@ import (
 
 type Config struct {
 	Server         ServerConfig         `yaml:"server"`
+	Storage        StorageConfig        `yaml:"storage"`
 	Database       DatabaseConfig       `yaml:"database"`
 	Log            LogConfig            `yaml:"log"`
 	Auth           AuthConfig           `yaml:"auth"`
@@ -24,6 +25,18 @@ type ServerConfig struct {
 	Address string `yaml:"address"`
 }
 
+// StorageConfig points at on-disk locations the server uses for cached
+// runtime artifacts. Currently:
+//   - DataDir hosts <DataDir>/playlist_covers/{playlist_id}.jpg (M7 #352).
+// More entries may grow here (e.g., transcode cache) without breaking
+// existing layouts.
+type StorageConfig struct {
+	// DataDir is the root for cached artifacts. Default: "./data" (relative
+	// to the working directory). Operators in production should set an
+	// absolute path persistent across restarts.
+	DataDir string `yaml:"data_dir"`
+}
+
 type DatabaseConfig struct {
 	URL string `yaml:"url"`
 }
@@ -82,6 +95,7 @@ type RecommendationConfig struct {
 func Default() Config {
 	return Config{
 		Server:   ServerConfig{Address: ":4533"},
+		Storage:  StorageConfig{DataDir: "./data"},
 		Database: DatabaseConfig{URL: ""},
 		Log:      LogConfig{Level: "info", Format: "text"},
 		Events: EventsConfig{
@@ -124,6 +138,9 @@ func applyEnv(cfg *Config) {
 	if v, ok := os.LookupEnv("SMARTMUSIC_SERVER_ADDRESS"); ok {
 		cfg.Server.Address = v
 	}
+	if v, ok := os.LookupEnv("SMARTMUSIC_STORAGE_DATA_DIR"); ok {
+		cfg.Storage.DataDir = v
+	}
 	if v, ok := os.LookupEnv("SMARTMUSIC_DATABASE_URL"); ok {
 		cfg.Database.URL = v
 	}

internal/server/server.go

@@ -70,8 +70,8 @@ type Server struct {
 	DataDir string
 }
 
-func New(logger *slog.Logger, pool *pgxpool.Pool, scanner ScanTrigger, subCfg subsonic.Config, eventsCfg config.EventsConfig, recCfg config.RecommendationConfig) *Server {
-	return &Server{Logger: logger, Pool: pool, Scanner: scanner, SubsonicCfg: subCfg, EventsCfg: eventsCfg, RecommendationCfg: recCfg}
+func New(logger *slog.Logger, pool *pgxpool.Pool, scanner ScanTrigger, subCfg subsonic.Config, eventsCfg config.EventsConfig, recCfg config.RecommendationConfig, dataDir string) *Server {
+	return &Server{Logger: logger, Pool: pool, Scanner: scanner, SubsonicCfg: subCfg, EventsCfg: eventsCfg, RecommendationCfg: recCfg, DataDir: dataDir}
 }
 
 func (s *Server) Router() http.Handler {