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_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")
+	}
+}