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

frontend/src/pages/InstanceGroupPage.tsx

@@ -195,8 +195,8 @@ export default function InstanceGroupPage() {
     setFormName(group.name);
     setFormDesc(group.description || '');
     setFormIsActive(group.is_active);
-    setReadonlyIds((group.readonly_instances || []).map((i: any) => String(i.id)));
-    setWriteIds((group.write_instances || []).map((i: any) => String(i.id)));
+    setReadonlyIds((group.readonly_instance_ids || []).map((i: any) => String(typeof i === 'object' ? i.id : i)));
+    setWriteIds((group.write_instance_ids || []).map((i: any) => String(typeof i === 'object' ? i.id : i)));
     setSelectedId(group.id);
     setShowEditModal(true);
   };
@@ -354,6 +354,8 @@ export default function InstanceGroupPage() {
             const sg = selectedGroup;
             const roInst = sg.readonly_instances || [];
             const wrInst = sg.write_instances || [];
+            const roCount = sg.readonly_instance_count ?? (sg.readonly_instance_ids || []).length;
+            const wrCount = sg.write_instance_count ?? (sg.write_instance_ids || []).length;
             return (
               <div className="space-y-4">
                 <Card className="dg-card">
@@ -366,8 +368,8 @@ export default function InstanceGroupPage() {
                         </div>
                         {sg.description && <p className="text-sm text-[var(--dg-text-secondary)] mt-1">{sg.description}</p>}
                         <div className="flex items-center gap-2 mt-2">
-                          <span className="px-2 py-0.5 text-xs rounded bg-emerald-500/15 text-emerald-400">{roInst.length} 只读实例</span>
-                          <span className="px-2 py-0.5 text-xs rounded bg-amber-500/15 text-amber-400">{wrInst.length} 写入实例</span>
+                          <span className="px-2 py-0.5 text-xs rounded bg-emerald-500/15 text-emerald-400">{roCount} 只读实例</span>
+                          <span className="px-2 py-0.5 text-xs rounded bg-amber-500/15 text-amber-400">{wrCount} 写入实例</span>
                           <span className="px-2 py-0.5 text-xs rounded bg-blue-500/15 text-blue-400">{sg.user_count || 0} 用户</span>
                         </div>
                       </div>