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/usagelog/usagelog.go

@@ -62,6 +62,8 @@ const (
 	FieldDurationMs = "duration_ms"
 	// FieldFirstTokenMs holds the string denoting the first_token_ms field in the database.
 	FieldFirstTokenMs = "first_token_ms"
+	// FieldUserAgent holds the string denoting the user_agent field in the database.
+	FieldUserAgent = "user_agent"
 	// FieldImageCount holds the string denoting the image_count field in the database.
 	FieldImageCount = "image_count"
 	// FieldImageSize holds the string denoting the image_size field in the database.
@@ -144,6 +146,7 @@ var Columns = []string{
 	FieldStream,
 	FieldDurationMs,
 	FieldFirstTokenMs,
+	FieldUserAgent,
 	FieldImageCount,
 	FieldImageSize,
 	FieldCreatedAt,
@@ -194,6 +197,8 @@ var (
 	DefaultBillingType int8
 	// DefaultStream holds the default value on creation for the "stream" field.
 	DefaultStream bool
+	// UserAgentValidator is a validator for the "user_agent" field. It is called by the builders before save.
+	UserAgentValidator func(string) error
 	// DefaultImageCount holds the default value on creation for the "image_count" field.
 	DefaultImageCount int
 	// ImageSizeValidator is a validator for the "image_size" field. It is called by the builders before save.
@@ -330,6 +335,11 @@ func ByFirstTokenMs(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldFirstTokenMs, opts...).ToFunc()
 }
 
+// ByUserAgent orders the results by the user_agent field.
+func ByUserAgent(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldUserAgent, opts...).ToFunc()
+}
+
 // ByImageCount orders the results by the image_count field.
 func ByImageCount(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldImageCount, opts...).ToFunc()