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/README.md

@@ -0,0 +1,16 @@
+# Game Group V2 Backend
+
+PocketBase BaaS 服务
+
+## 启动
+
+```bash
+# 方式1: Docker
+docker-compose up -d
+
+# 方式2: 直接运行
+./pocketbase serve --env .env
+```
+
+访问: http://localhost:8090/_/
+管理后台: http://localhost:8090/_/ (admin/admin)