feat(backend): add user custom attributes system

Add a flexible user attribute system that allows admins to define
custom fields for users (text, textarea, number, email, url, date,
select, multi_select types).

- Add Ent schemas for UserAttributeDefinition and UserAttributeValue
- Add service layer with validation logic
- Add repository layer with batch operations support
- Add admin API endpoints for CRUD and reorder operations
- Add batch API for loading attribute values for multiple users
- Add database migration (018_user_attributes.sql)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/server/routes/admin.go

@@ -54,6 +54,9 @@ func RegisterAdminRoutes(
 
 		// 使用记录管理
 		registerUsageRoutes(admin, h)
+
+		// 用户属性管理
+		registerUserAttributeRoutes(admin, h)
 	}
 }
 
@@ -82,6 +85,10 @@ func registerUserManagementRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		users.POST("/:id/balance", h.Admin.User.UpdateBalance)
 		users.GET("/:id/api-keys", h.Admin.User.GetUserAPIKeys)
 		users.GET("/:id/usage", h.Admin.User.GetUserUsage)
+
+		// User attribute values
+		users.GET("/:id/attributes", h.Admin.UserAttribute.GetUserAttributes)
+		users.PUT("/:id/attributes", h.Admin.UserAttribute.UpdateUserAttributes)
 	}
 }
 
@@ -242,3 +249,15 @@ func registerUsageRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		usage.GET("/search-api-keys", h.Admin.Usage.SearchApiKeys)
 	}
 }
+
+func registerUserAttributeRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
+	attrs := admin.Group("/user-attributes")
+	{
+		attrs.GET("", h.Admin.UserAttribute.ListDefinitions)
+		attrs.POST("", h.Admin.UserAttribute.CreateDefinition)
+		attrs.POST("/batch", h.Admin.UserAttribute.GetBatchUserAttributes)
+		attrs.PUT("/reorder", h.Admin.UserAttribute.ReorderDefinitions)
+		attrs.PUT("/:id", h.Admin.UserAttribute.UpdateDefinition)
+		attrs.DELETE("/:id", h.Admin.UserAttribute.DeleteDefinition)
+	}
+}