feat: add SyncAll periodic sync pipeline and wire Navidrome client into App

internal/navidrome/client.go

@@ -47,6 +47,19 @@ func NewClient(cfg config.NavidromeConfig) (*NavidromeClient, error) {
 	return &NavidromeClient{client: client}, nil
 }
 
+// NewClientUnauthenticated builds a NavidromeClient without contacting the
+// server. It is intended for dependency injection in tests (where the
+// navidromeClientFactory seam in main is overridden) and for callers that want
+// to defer or skip authentication. Production wiring should prefer NewClient.
+func NewClientUnauthenticated(cfg config.NavidromeConfig) *NavidromeClient {
+	return &NavidromeClient{client: &subsonic.Client{
+		Client:     &http.Client{Timeout: 30 * time.Second},
+		BaseUrl:    cfg.URL,
+		User:       cfg.User,
+		ClientName: "naviwatcher",
+	}}
+}
+
 // Ping checks connectivity to the Navidrome server.
 // Returns nil if the server is reachable and responds with a valid Subsonic OK status.
 func (nc *NavidromeClient) Ping() error {