fix: restore Dev PocketBase port mapping to 8090

Was 8711:8090 but Dev PB should be on host port 8090 per CLAUDE.md
environment table. UAT remains on 8712.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.backend.yml

@@ -3,7 +3,7 @@ services:
     image: ghcr.io/muchobien/pocketbase:0.22.4
     container_name: gamegroup-pb
     ports:
-      - "8711:8090"
+      - "8090:8090"
     volumes:
       - ./backend/pb_data:/pb_data
       - ./backend/pb_migrations:/pb_migrations