feat(usage-log): 增加请求 User-Agent 记录

在使用记录中添加 user_agent 字段，用于记录 API 请求的 User-Agent 头信息，
便于分析客户端类型和调试。

变更内容：
- 新增数据库迁移 028_add_usage_logs_user_agent.sql
- 更新 UsageLog 模型和 Ent Schema 添加 user_agent 字段
- 更新 Repository 层的 Create 和 scanUsageLog 方法
- 更新 RecordUsageInput 结构体支持传入 UserAgent
- 更新 Claude/OpenAI/Gemini 三个网关 Handler 传递 UserAgent

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

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

backend/ent/runtime/runtime.go

@@ -521,16 +521,20 @@ func init() {
 	usagelogDescStream := usagelogFields[21].Descriptor()
 	// usagelog.DefaultStream holds the default value on creation for the stream field.
 	usagelog.DefaultStream = usagelogDescStream.Default.(bool)
+	// usagelogDescUserAgent is the schema descriptor for user_agent field.
+	usagelogDescUserAgent := usagelogFields[24].Descriptor()
+	// usagelog.UserAgentValidator is a validator for the "user_agent" field. It is called by the builders before save.
+	usagelog.UserAgentValidator = usagelogDescUserAgent.Validators[0].(func(string) error)
 	// usagelogDescImageCount is the schema descriptor for image_count field.
-	usagelogDescImageCount := usagelogFields[24].Descriptor()
+	usagelogDescImageCount := usagelogFields[25].Descriptor()
 	// usagelog.DefaultImageCount holds the default value on creation for the image_count field.
 	usagelog.DefaultImageCount = usagelogDescImageCount.Default.(int)
 	// usagelogDescImageSize is the schema descriptor for image_size field.
-	usagelogDescImageSize := usagelogFields[25].Descriptor()
+	usagelogDescImageSize := usagelogFields[26].Descriptor()
 	// usagelog.ImageSizeValidator is a validator for the "image_size" field. It is called by the builders before save.
 	usagelog.ImageSizeValidator = usagelogDescImageSize.Validators[0].(func(string) error)
 	// usagelogDescCreatedAt is the schema descriptor for created_at field.
-	usagelogDescCreatedAt := usagelogFields[26].Descriptor()
+	usagelogDescCreatedAt := usagelogFields[27].Descriptor()
 	// usagelog.DefaultCreatedAt holds the default value on creation for the created_at field.
 	usagelog.DefaultCreatedAt = usagelogDescCreatedAt.Default.(func() time.Time)
 	userMixin := schema.User{}.Mixin()