test: notification handler + gotify client unit tests

- 24 tests covering all X-Resource routes
- Mock Gotify server for message/application CRUD
- Error cases: nil gotify, invalid JSON, missing fields, 429, 500
- Fix readError() to handle io.ReadAll failure
- GotifyClient direct tests for all 7 methods

internal/notification/gotify.go

@@ -252,6 +252,9 @@ func (c *GotifyClient) DeleteApplication(id int) error {
 // --- Helpers ---
 
 func (c *GotifyClient) readError(resp *http.Response) error {
-	body, _ := io.ReadAll(resp.Body)
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return fmt.Errorf("gotify API error (HTTP %d): failed to read body: %w", resp.StatusCode, err)
+	}
 	return fmt.Errorf("gotify API error (HTTP %d): %s", resp.StatusCode, string(body))
 }