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/types.go

@@ -1,6 +1,7 @@
 package serviceadapter
 
 import (
+	"net/http"
 	"time"
 )
 
@@ -49,11 +50,14 @@ type Status struct {
 }
 
 // ServiceAdapter is a gateway service adapter.
+// When Handler is set, the dispatcher routes directly to the internal handler
+// instead of reverse-proxying to Spec.Upstream.URL.
 type ServiceAdapter struct {
-	Name      string // namespace/name
-	Namespace string
+	Name        string // namespace/name
+	Namespace   string
 	ServiceName string
-	Spec      Spec
-	Status    Status
-	CreatedAt time.Time
+	Spec        Spec
+	Status      Status
+	CreatedAt   time.Time
+	Handler     http.Handler `json:"-" yaml:"-"` // internal handler (skip serialization)
 }