feat: Gotify CRUD + internal handler dispatch for notification service

- Add internal handler support to ServiceAdapter (Handler field)
- Dispatcher routes to internal handler when set (no reverse proxy)
- GotifyClient: full CRUD (send/list/delete messages, CRUD applications)
- Refactor notification handler: route by X-Resource header, not body format
- Register notification as ServiceAdapter with auth required
- Resources: send-email, send-message, list-messages, delete-message,
  delete-all-messages, list-applications, create-application, delete-application
- Update examples: sendmsg-email.sh, gotify-crud.sh
- Env vars: GOTIFY_URL, GOTIFY_APP_TOKEN, GOTIFY_CLIENT_TOKEN

internal/serviceadapter/router.go

@@ -80,6 +80,14 @@ func (d *Dispatcher) Dispatch(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	// Internal handler: dispatch directly without reverse proxy
+	if adapter.Handler != nil {
+		// Set X-Upstream-Path so the handler knows which method was matched
+		r.Header.Set("X-Upstream-Path", method.UpstreamPath)
+		adapter.Handler.ServeHTTP(w, r)
+		return
+	}
+
 	upstreamURL := adapter.Spec.Upstream.URL
 	if strings.HasPrefix(upstreamURL, "grpc://") {
 		d.dispatchGRPC(w, r, upstreamURL, method, adapter)