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.

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 {