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/instance_extra.go

@@ -69,9 +69,15 @@ func ListDatabases(c *gin.Context) {
 		c.JSON(http.StatusNotFound, gin.H{"detail": "Instance not found"})
 		return
 	}
-	// TODO: actual database connection to list databases
-	// Return placeholder for now
-	c.JSON(http.StatusOK, []string{"information_schema"})
+	databases, err := listDatabases(&instance)
+	if err != nil {
+		c.JSON(http.StatusOK, []string{})
+		return
+	}
+	if databases == nil {
+		databases = []string{}
+	}
+	c.JSON(http.StatusOK, databases)
 }
 
 // GET /api/v1/instances/:id/databases/:db/tables
@@ -83,16 +89,47 @@ func ListTables(c *gin.Context) {
 		c.JSON(http.StatusNotFound, gin.H{"detail": "Instance not found"})
 		return
 	}
-	// TODO: actual database connection to list tables
-	_ = dbName
-	c.JSON(http.StatusOK, []interface{}{})
+	tables, err := listTables(&instance, dbName)
+	if err != nil {
+		c.JSON(http.StatusOK, []interface{}{})
+		return
+	}
+	if tables == nil {
+		tables = []map[string]interface{}{}
+	}
+	c.JSON(http.StatusOK, tables)
 }
 
 // POST /api/v1/instances/test-connection (new instance, not saved yet)
 func TestNewConnection(c *gin.Context) {
-	// TODO: actual connection test
-	c.JSON(http.StatusOK, gin.H{
-		"success": true,
-		"message": "Connection test passed (placeholder)",
-	})
+	var req struct {
+		DBType   string `json:"db_type"`
+		Host     string `json:"host"`
+		Port     int    `json:"port"`
+		Username string `json:"username"`
+		Password string `json:"password"`
+		Database string `json:"database"`
+	}
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": "Invalid request"})
+		return
+	}
+
+	instance := &model.Instance{
+		DBType:   req.DBType,
+		Host:     req.Host,
+		Port:     req.Port,
+		Username: req.Username,
+		Password: req.Password,
+	}
+	if req.Database != "" {
+		instance.Database = &req.Database
+	}
+
+	success, message := testDBConnection(instance)
+	if success {
+		c.JSON(http.StatusOK, gin.H{"success": true, "message": message})
+	} else {
+		c.JSON(http.StatusBadRequest, gin.H{"success": false, "message": message})
+	}
 }