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

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

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