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/handler/instance.go

@@ -0,0 +1,153 @@
+package handler
+
+import (
+	"net/http"
+	"strconv"
+
+	"sql-platform/internal/middleware"
+	"sql-platform/internal/model"
+
+	"github.com/gin-gonic/gin"
+)
+
+// GET /api/v1/instances
+func ListInstances(c *gin.Context) {
+	userID, _, role := middleware.GetCurrentUser(c)
+	
+	var instances []model.Instance
+	query := model.DB.Order("id")
+	
+	// developer only sees instances in their groups
+	if role == "developer" {
+		var groupIDs []int
+		model.DB.Model(&model.UserGroup{}).Where("user_id = ?", userID).Pluck("group_id", &groupIDs)
+		if len(groupIDs) > 0 {
+			var instanceIDs []int
+			model.DB.Model(&model.InstanceGroupMember{}).Where("group_id IN ?", groupIDs).Pluck("instance_id", &instanceIDs)
+			if len(instanceIDs) > 0 {
+				query = query.Where("id IN ?", instanceIDs)
+			} else {
+				c.JSON(http.StatusOK, []model.Instance{})
+				return
+			}
+		}
+	}
+	
+	if err := query.Find(&instances).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"detail": err.Error()})
+		return
+	}
+	
+	// mask passwords in response
+	for i := range instances {
+		instances[i].Password = "******"
+	}
+	c.JSON(http.StatusOK, instances)
+}
+
+// GET /api/v1/instances/:id
+func GetInstance(c *gin.Context) {
+	id := c.Param("id")
+	var instance model.Instance
+	if err := model.DB.First(&instance, id).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"detail": "Instance not found"})
+		return
+	}
+	instance.Password = "******"
+	c.JSON(http.StatusOK, instance)
+}
+
+// POST /api/v1/instances
+func CreateInstance(c *gin.Context) {
+	var instance model.Instance
+	if err := c.ShouldBindJSON(&instance); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": err.Error()})
+		return
+	}
+	if err := model.DB.Create(&instance).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"detail": err.Error()})
+		return
+	}
+	instance.Password = "******"
+	c.JSON(http.StatusCreated, instance)
+}
+
+// PUT /api/v1/instances/:id
+func UpdateInstance(c *gin.Context) {
+	id := c.Param("id")
+	var instance model.Instance
+	if err := model.DB.First(&instance, id).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"detail": "Instance not found"})
+		return
+	}
+	
+	var input map[string]interface{}
+	if err := c.ShouldBindJSON(&input); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": err.Error()})
+		return
+	}
+	
+	// if password is masked, don't update it
+	if pw, ok := input["password"].(string); ok && pw == "******" {
+		delete(input, "password")
+	}
+	
+	if err := model.DB.Model(&instance).Updates(input).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"detail": err.Error()})
+		return
+	}
+	
+	model.DB.First(&instance, id)
+	instance.Password = "******"
+	c.JSON(http.StatusOK, instance)
+}
+
+// DELETE /api/v1/instances/:id
+func DeleteInstance(c *gin.Context) {
+	id := c.Param("id")
+	if err := model.DB.Delete(&model.Instance{}, id).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"detail": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"message": "Instance deleted"})
+}
+
+// POST /api/v1/instances/:id/test-connection
+func TestConnection(c *gin.Context) {
+	id := c.Param("id")
+	var instance model.Instance
+	if err := model.DB.First(&instance, id).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"detail": "Instance not found"})
+		return
+	}
+	
+	// TODO: actual DB connection test
+	c.JSON(http.StatusOK, gin.H{
+		"success": true,
+		"message": "Connection test passed (placeholder)",
+	})
+}
+
+// GET /api/v1/instances/stats
+func InstanceStats(c *gin.Context) {
+	var total int64
+	model.DB.Model(&model.Instance{}).Count(&total)
+	
+	type typeCount struct {
+		DBType string `json:"db_type"`
+		Count  int64  `json:"count"`
+	}
+	var byType []typeCount
+	model.DB.Model(&model.Instance{}).Select("db_type, count(*) as count").Group("db_type").Scan(&byType)
+	
+	c.JSON(http.StatusOK, gin.H{
+		"total":  total,
+		"by_type": byType,
+	})
+}
+
+// Helper: parse id param to int
+func parseID(s string) (int, bool) {
+	id, err := strconv.Atoi(s)
+	return id, err == nil
+}