feat: initialize PocketBase backend with migrations

- Created backend directory structure
- Added .env configuration for PocketBase
- Added initial migration with users, groups, games, teamSessions, invitations collections
- Added docker-compose.yml for containerized deployment
- Added README.md with setup instructions

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/docker-compose.yml

@@ -0,0 +1,14 @@
+version: '3.8'
+
+services:
+  pocketbase:
+    image: ghcr.io/muchobien/pocketbase:latest
+    container_name: gamegroup-pb
+    ports:
+      - "${PB_PORT:-8090}:8090"
+    volumes:
+      - ./pb_data:/pb_data
+      - ./pb_migrations:/pb_migrations
+    environment:
+      - GO_ENV=production
+    restart: unless-stopped