feat: wire main loop with periodic sync+scan and sync_interval config

Replace compute-only run() with an immediate sync+scan followed by a
ticker-driven periodic loop, add the sync.interval config field (default
6h) with defaults and validation, and add tests for the scheduling logic.

internal/config/config.go

@@ -15,6 +15,7 @@ type Config struct {
 	MusicBrainz MusicBrainzConfig `yaml:"musicbrainz"`
 	Telegram    TelegramConfig    `yaml:"telegram"`
 	Scanner     ScannerConfig     `yaml:"scanner"`
+	Sync        SyncConfig        `yaml:"sync"`
 }
 
 // ServerConfig holds HTTP server settings.
@@ -46,6 +47,15 @@ type TelegramConfig struct {
 	CronSchedule string `yaml:"cron_schedule"`
 }
 
+// SyncConfig holds periodic sync pipeline settings.
+type SyncConfig struct {
+	Interval time.Duration `yaml:"interval"`
+}
+
+// DefaultSyncInterval is the default period between full sync+scan runs when
+// sync.interval is not specified in the config file.
+const DefaultSyncInterval = 6 * time.Hour
+
 // ScannerConfig holds scanner engine parameters.
 //
 // Type filtering is handled in musicbrainz/api.go, not here: only Album/Single/EP
@@ -95,6 +105,9 @@ func applyDefaults(cfg *Config) {
 	if cfg.MusicBrainz.CacheTTL == 0 {
 		cfg.MusicBrainz.CacheTTL = 24 * time.Hour
 	}
+	if cfg.Sync.Interval == 0 {
+		cfg.Sync.Interval = DefaultSyncInterval
+	}
 }
 
 // validate checks that required fields are set and values are within acceptable ranges.
@@ -117,5 +130,8 @@ func validate(cfg *Config) error {
 	if cfg.MusicBrainz.UserAgent == "" {
 		return fmt.Errorf("musicbrainz.user_agent is required")
 	}
+	if cfg.Sync.Interval <= 0 {
+		return fmt.Errorf("sync.interval must be positive, got %v", cfg.Sync.Interval)
+	}
 	return nil
 }