fix(nginx): internalPort as string + add missing source field

- internalPort: 80 (number) -> "80" (string) to match Runtipi schema
- Added missing source field required for app discovery

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/nginx/docker-compose.json

@@ -4,7 +4,7 @@
       "name": "nginx",
       "image": "nginx:latest",
       "isMain": true,
-      "internalPort": 80,
+      "internalPort": "80",
       "volumes": [
         {
           "hostPath": "${APP_DATA_DIR}/conf.d",