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
 }

internal/config/config_test.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+	"time"
 )
 
 func TestLoadConfig_Valid(t *testing.T) {
@@ -338,3 +339,83 @@ func TestValidate_BoundaryPort(t *testing.T) {
 		})
 	}
 }
+
+func TestLoadConfig_SyncIntervalDefault(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+
+	yaml := `
+navidrome:
+  url: "http://localhost:4533"
+  user: "u"
+  password: "p"
+
+musicbrainz:
+  user_agent: "NaviWatcher/1.0 ( test@example.com )"
+`
+	if err := os.WriteFile(path, []byte(yaml), 0644); err != nil {
+		t.Fatalf("failed to write config: %v", err)
+	}
+
+	cfg, err := LoadConfig(path)
+	if err != nil {
+		t.Fatalf("LoadConfig returned error: %v", err)
+	}
+	if cfg.Sync.Interval != DefaultSyncInterval {
+		t.Errorf("expected default sync interval %v, got %v", DefaultSyncInterval, cfg.Sync.Interval)
+	}
+}
+
+func TestLoadConfig_SyncIntervalParsed(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+
+	yaml := `
+navidrome:
+  url: "http://localhost:4533"
+  user: "u"
+  password: "p"
+
+musicbrainz:
+  user_agent: "NaviWatcher/1.0 ( test@example.com )"
+
+sync:
+  interval: 30m
+`
+	if err := os.WriteFile(path, []byte(yaml), 0644); err != nil {
+		t.Fatalf("failed to write config: %v", err)
+	}
+
+	cfg, err := LoadConfig(path)
+	if err != nil {
+		t.Fatalf("LoadConfig returned error: %v", err)
+	}
+	if cfg.Sync.Interval != 30*time.Minute {
+		t.Errorf("expected sync interval 30m, got %v", cfg.Sync.Interval)
+	}
+}
+
+func TestLoadConfig_InvalidSyncInterval(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+
+	yaml := `
+navidrome:
+  url: "http://localhost:4533"
+  user: "u"
+  password: "p"
+
+musicbrainz:
+  user_agent: "NaviWatcher/1.0 ( test@example.com )"
+
+sync:
+  interval: -1s
+`
+	if err := os.WriteFile(path, []byte(yaml), 0644); err != nil {
+		t.Fatalf("failed to write config: %v", err)
+	}
+
+	if _, err := LoadConfig(path); err == nil {
+		t.Fatal("expected error for negative sync interval, got nil")
+	}
+}

internal/musicbrainz/syncall.go

@@ -58,6 +58,12 @@ func (s *albumSyncer) SyncAlbums(ctx context.Context, db *database.DB) error {
 	return s.syncAlbums(ctx, db)
 }
 
+// NewAlbumSyncer adapts the given SyncAlbums function (typically
+// navidrome.SyncAlbums) into an AlbumSyncer for injection into SyncAll.
+func NewAlbumSyncer(syncAlbums func(ctx context.Context, db *database.DB) error) AlbumSyncer {
+	return &albumSyncer{syncAlbums: syncAlbums}
+}
+
 // SyncAll orchestrates the data pipeline for every monitored artist:
 //  1. MusicBrainz artist-ID resolution — for each artist with no cached MBID,
 //     resolve it by name and persist it on the artist_settings row. Artists that