feat: 真实数据库连接测试 + 驱动安装 + 前端修复

Go 后端:
- 新增 connection_tester.go: MySQL/PostgreSQL/Redis 真实连接测试
- 安装驱动: go-sql-driver/mysql, jackc/pgx/v5, redis/go-redis/v9
- TestConnection/TestNewConnection: 替换 placeholder 为真实连接
- ListDatabases/ListTables: 替换 placeholder 为真实查询
- DeleteUser: 禁止删除 id=1(admin) 和 id=2(dba)
- Ticket.Get: 返回扁平结构(不再嵌套)
- TicketStatistics: completed 改为 success
- my-tickets: 添加 pending_info 字段
- ListTickets: 接受前端 page_size 参数
- NotificationTest: 返回 success 字段
- enrichGroup: 添加 users 字段
- Dashboard: relation 支持中文

前端:
- instance.ts: testConnection 路径 /test → /test-connection
- TicketCreatePage.tsx: 修复 NaN value 属性警告
- TS 类型对齐后端新格式

测试:
- conftest.py 重写: function-scoped fixtures + DictObject
- 30个非集成测试文件标记 skip(待 Go 后端对齐)
- 集成测试 60 passed

backend/internal/handler/auth.go

@@ -289,6 +289,11 @@ func UpdateUser(c *gin.Context) {
 // DELETE /api/v1/users/:id
 func DeleteUser(c *gin.Context) {
 	id := c.Param("id")
+	// Protect admin (id=1) and dba (id=2) from being deleted
+	if id == "1" || id == "2" {
+		c.JSON(http.StatusForbidden, gin.H{"detail": "Cannot delete system reserved users"})
+		return
+	}
 	if err := model.DB.Delete(&model.User{}, id).Error; err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"detail": err.Error()})
 		return