fix: implement upsert behavior for service creation

When a service is republished (e.g., for TTL refresh), the old service
is now deleted before creating a new one, preventing UNIQUE constraint
errors on (username, service_fqn).

Changes:
- Query for existing service before creation
- Delete existing service if found
- Create new service with same username/serviceFqn

This enables the client's TTL auto-refresh feature to work correctly.

src/app.ts

@@ -61,7 +61,7 @@ export function createApp(storage: Storage, config: Config) {
 
   /**
    * POST /register
-   * Register a new peer (still needed for peer ID generation)
+   * Register a new peer
    */
   app.post('/register', async (c) => {
     try {