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/dashboard.go

@@ -0,0 +1,68 @@
+package handler
+
+import (
+	"net/http"
+
+	"sql-platform/internal/middleware"
+	"sql-platform/internal/model"
+
+	"github.com/gin-gonic/gin"
+)
+
+// GET /api/v1/dashboard/stats
+func DashboardStats(c *gin.Context) {
+	var ticketCount, queryCount, instanceCount int64
+	model.DB.Model(&model.Ticket{}).Count(&ticketCount)
+	model.DB.Model(&model.QueryHistory{}).Count(&queryCount)
+	model.DB.Model(&model.Instance{}).Count(&instanceCount)
+	
+	c.JSON(http.StatusOK, gin.H{
+		"ticket_count":  ticketCount,
+		"query_count":   queryCount,
+		"instance_count": instanceCount,
+	})
+}
+
+// GET /api/v1/dashboard/my-tickets
+func MyTickets(c *gin.Context) {
+	userID, _, role := middleware.GetCurrentUser(c)
+	
+	var tickets []model.Ticket
+	query := model.DB.Order("id DESC").Limit(10)
+	
+	switch role {
+	case "admin":
+		// Admin sees all pending/executing tickets
+		query = query.Where("status IN ?", []string{"pending", "reviewing", "executing"})
+	case "dba", "auditor":
+		// DBA sees their own + pending approval
+		query = query.Where(
+			"(applicant_id = ? AND status IN ?) OR (id IN (SELECT ticket_id FROM ticket_approvals WHERE approver_id = ?) AND status IN ?)",
+			userID, []string{"pending", "executing"},
+			userID, []string{"pending", "reviewing", "executing"},
+		)
+	default: // developer
+		query = query.Where("applicant_id = ?", userID)
+	}
+	
+	query.Find(&tickets)
+	
+	type ticketWithRelation struct {
+		model.Ticket
+		Relation string `json:"relation"` // "submitted" or "pending_approval"
+	}
+	
+	var result []ticketWithRelation
+	for _, t := range tickets {
+		rel := "submitted"
+		if t.ApplicantID != userID {
+			rel = "pending_approval"
+		}
+		result = append(result, ticketWithRelation{Ticket: t, Relation: rel})
+	}
+	
+	if result == nil {
+		result = []ticketWithRelation{}
+	}
+	c.JSON(http.StatusOK, result)
+}