feat: v0.3.1 — Go/Gin 后端重写 + gh-ost 集成

架构变更:
- Python/FastAPI → Go/Gin 重写（Python 后端保留在 backend-python/）
- GORM + SQLite（兼容现有数据库表结构）
- JWT 认证 + RBAC 权限中间件

Go 后端结构:
- cmd/server/main.go — 入口，端口 9000
- config/config.go — 环境变量配置
- internal/model/ — GORM 模型（30张表，完全兼容 SQLite）
- internal/handler/ — auth/instance/ticket/dashboard/ghost handler
- internal/middleware/ — JWT 认证 + CORS
- internal/ghost/ — gh-ost 集成模块（同步/异步执行，任务管理）
- router/router.go — API 路由注册（/api/v1/...）

gh-ost 集成:
- RunGhOst() 同步执行
- RunGhOstAsync() 异步执行 + 进度追踪
- TaskManager 管理多个并发任务
- API: POST /ghost/dry-run, /ghost/execute, GET /ghost/status/:taskId

核心 API 已对齐:
- POST /auth/login (JWT)
- GET /dashboard/stats, /dashboard/my-tickets
- GET /tickets/statistics, CRUD + approve/execute
- GET/POST/PUT/DELETE /instances
- GET /users, /audit-rules, /announcements, /settings

已验证:
- 登录 admin/admin123 ✅
- Dashboard 99工单/110查询/4实例 ✅
- 工单统计按角色过滤 ✅

backend/internal/middleware/cors.go

@@ -0,0 +1,16 @@
+package middleware
+
+import (
+	"github.com/gin-contrib/cors"
+	"github.com/gin-gonic/gin"
+)
+
+func CORSMiddleware() gin.HandlerFunc {
+	return cors.New(cors.Config{
+		AllowOrigins:     []string{"http://localhost:5173", "http://127.0.0.1:5173"},
+		AllowMethods:     []string{"GET", "POST", "PUT", "DELETE", "PATCH", "OPTIONS"},
+		AllowHeaders:     []string{"Origin", "Content-Type", "Authorization"},
+		ExposeHeaders:    []string{"Content-Length"},
+		AllowCredentials: true,
+	})
+}