fix(scanner): handle nil mainCtx in Watcher to prevent panic

Signed-off-by: Deluan <deluan@navidrome.org>

scanner/watcher.go

@@ -145,6 +145,12 @@ func (w *watcher) Watch(ctx context.Context, lib *model.Library) error {
 	w.mu.Lock()
 	defer w.mu.Unlock()
 
+	// If Run() hasn't been called yet, mainCtx will be nil - skip watching
+	if w.mainCtx == nil {
+		log.Debug(ctx, "Watcher not started yet, skipping watch for library", "libraryID", lib.ID, "name", lib.Name)
+		return nil
+	}
+
 	// Stop existing watcher if any
 	if existingInstance, exists := w.libraryWatchers[lib.ID]; exists {
 		log.Debug(ctx, "Stopping existing watcher before starting new one", "libraryID", lib.ID, "name", lib.Name)

scanner/watcher_test.go

@@ -54,6 +54,15 @@ var _ = Describe("Watcher", func() {
 		}
 	})
 
+	Describe("Watch before Run", func() {
+		It("returns nil and does not panic when mainCtx is nil", func() {
+			w.mainCtx = nil
+			err := w.Watch(ctx, lib)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(w.libraryWatchers).To(BeEmpty())
+		})
+	})
+
 	Describe("Target Collection and Deduplication", func() {
 		BeforeEach(func() {
 			// Start watcher in background