Add upsert behavior to service creation

- Delete existing service before creating new one
- Prevents UNIQUE constraint error on (username, service_fqn)
- Enables seamless service republishing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/app.ts

@@ -217,6 +217,15 @@ export function createApp(storage: Storage, config: Config) {
         return c.json({ error: 'Invalid signature for username' }, 403);
       }
 
+      // Delete existing service if one exists (upsert behavior)
+      const existingUuid = await storage.queryService(username, serviceFqn);
+      if (existingUuid) {
+        const existingService = await storage.getServiceByUuid(existingUuid);
+        if (existingService) {
+          await storage.deleteService(existingService.id, username);
+        }
+      }
+
       // Validate SDP
       if (typeof sdp !== 'string' || sdp.length === 0) {
         return c.json({ error: 'Invalid SDP' }, 400);