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/service/instance_service.go

@@ -0,0 +1,93 @@
+package service
+
+import (
+	"errors"
+
+	"gorm.io/gorm"
+	"sql-platform/internal/model"
+)
+
+type InstanceService struct {
+	DB *gorm.DB
+}
+
+func NewInstanceService(db *gorm.DB) *InstanceService {
+	return &InstanceService{DB: db}
+}
+
+// CreateInstance creates a new database instance
+func (s *InstanceService) CreateInstance(instance *model.Instance) error {
+	return s.DB.Create(instance).Error
+}
+
+// GetInstanceByID retrieves an instance by ID
+func (s *InstanceService) GetInstanceByID(id uint) (*model.Instance, error) {
+	var instance model.Instance
+	if err := s.DB.First(&instance, id).Error; err != nil {
+		return nil, errors.New("instance not found")
+	}
+	return &instance, nil
+}
+
+// ListInstances returns paginated instances
+func (s *InstanceService) ListInstances(page, pageSize int, dbType, env string) ([]model.Instance, int64, error) {
+	var instances []model.Instance
+	var total int64
+
+	db := s.DB.Model(&model.Instance{})
+	if dbType != "" {
+		db = db.Where("db_type = ?", dbType)
+	}
+	if env != "" {
+		db = db.Where("env = ?", env)
+	}
+
+	db.Count(&total)
+	offset := (page - 1) * pageSize
+	if err := db.Offset(offset).Limit(pageSize).Order("created_at DESC").Find(&instances).Error; err != nil {
+		return nil, 0, err
+	}
+
+	return instances, total, nil
+}
+
+// UpdateInstance updates an instance
+func (s *InstanceService) UpdateInstance(id uint, updates map[string]interface{}) error {
+	result := s.DB.Model(&model.Instance{}).Where("id = ?", id).Updates(updates)
+	if result.RowsAffected == 0 {
+		return errors.New("instance not found")
+	}
+	return result.Error
+}
+
+// DeleteInstance soft-deletes an instance
+func (s *InstanceService) DeleteInstance(id uint) error {
+	return s.DB.Delete(&model.Instance{}, id).Error
+}
+
+// TestConnection tests the database connection
+func (s *InstanceService) TestConnection(instance *model.Instance) error {
+	// TODO: Implement actual connection test based on db_type
+	return nil
+}
+
+// ListInstancesByIDs returns instances matching the given IDs
+func (s *InstanceService) ListInstancesByIDs(ids []uint) ([]model.Instance, error) {
+	var instances []model.Instance
+	if err := s.DB.Where("id IN ?", ids).Find(&instances).Error; err != nil {
+		return nil, err
+	}
+	return instances, nil
+}
+
+// GetInstanceGroups returns all instance groups
+func (s *InstanceService) GetInstanceGroups() ([]model.InstanceGroup, error) {
+	var groups []model.InstanceGroup
+	err := s.DB.Find(&groups).Error
+	return groups, err
+}
+
+// CreateInstanceGroup creates a new instance group
+func (s *InstanceService) CreateInstanceGroup(group *model.InstanceGroup) error {
+	return s.DB.Create(group).Error
+}