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_update.go

@@ -504,6 +504,26 @@ func (_u *UsageLogUpdate) ClearFirstTokenMs() *UsageLogUpdate {
 	return _u
 }
 
+// SetUserAgent sets the "user_agent" field.
+func (_u *UsageLogUpdate) SetUserAgent(v string) *UsageLogUpdate {
+	_u.mutation.SetUserAgent(v)
+	return _u
+}
+
+// SetNillableUserAgent sets the "user_agent" field if the given value is not nil.
+func (_u *UsageLogUpdate) SetNillableUserAgent(v *string) *UsageLogUpdate {
+	if v != nil {
+		_u.SetUserAgent(*v)
+	}
+	return _u
+}
+
+// ClearUserAgent clears the value of the "user_agent" field.
+func (_u *UsageLogUpdate) ClearUserAgent() *UsageLogUpdate {
+	_u.mutation.ClearUserAgent()
+	return _u
+}
+
 // SetImageCount sets the "image_count" field.
 func (_u *UsageLogUpdate) SetImageCount(v int) *UsageLogUpdate {
 	_u.mutation.ResetImageCount()
@@ -644,6 +664,11 @@ func (_u *UsageLogUpdate) check() error {
 			return &ValidationError{Name: "model", err: fmt.Errorf(`ent: validator failed for field "UsageLog.model": %w`, err)}
 		}
 	}
+	if v, ok := _u.mutation.UserAgent(); ok {
+		if err := usagelog.UserAgentValidator(v); err != nil {
+			return &ValidationError{Name: "user_agent", err: fmt.Errorf(`ent: validator failed for field "UsageLog.user_agent": %w`, err)}
+		}
+	}
 	if v, ok := _u.mutation.ImageSize(); ok {
 		if err := usagelog.ImageSizeValidator(v); err != nil {
 			return &ValidationError{Name: "image_size", err: fmt.Errorf(`ent: validator failed for field "UsageLog.image_size": %w`, err)}
@@ -784,6 +809,12 @@ func (_u *UsageLogUpdate) sqlSave(ctx context.Context) (_node int, err error) {
 	if _u.mutation.FirstTokenMsCleared() {
 		_spec.ClearField(usagelog.FieldFirstTokenMs, field.TypeInt)
 	}
+	if value, ok := _u.mutation.UserAgent(); ok {
+		_spec.SetField(usagelog.FieldUserAgent, field.TypeString, value)
+	}
+	if _u.mutation.UserAgentCleared() {
+		_spec.ClearField(usagelog.FieldUserAgent, field.TypeString)
+	}
 	if value, ok := _u.mutation.ImageCount(); ok {
 		_spec.SetField(usagelog.FieldImageCount, field.TypeInt, value)
 	}
@@ -1433,6 +1464,26 @@ func (_u *UsageLogUpdateOne) ClearFirstTokenMs() *UsageLogUpdateOne {
 	return _u
 }
 
+// SetUserAgent sets the "user_agent" field.
+func (_u *UsageLogUpdateOne) SetUserAgent(v string) *UsageLogUpdateOne {
+	_u.mutation.SetUserAgent(v)
+	return _u
+}
+
+// SetNillableUserAgent sets the "user_agent" field if the given value is not nil.
+func (_u *UsageLogUpdateOne) SetNillableUserAgent(v *string) *UsageLogUpdateOne {
+	if v != nil {
+		_u.SetUserAgent(*v)
+	}
+	return _u
+}
+
+// ClearUserAgent clears the value of the "user_agent" field.
+func (_u *UsageLogUpdateOne) ClearUserAgent() *UsageLogUpdateOne {
+	_u.mutation.ClearUserAgent()
+	return _u
+}
+
 // SetImageCount sets the "image_count" field.
 func (_u *UsageLogUpdateOne) SetImageCount(v int) *UsageLogUpdateOne {
 	_u.mutation.ResetImageCount()
@@ -1586,6 +1637,11 @@ func (_u *UsageLogUpdateOne) check() error {
 			return &ValidationError{Name: "model", err: fmt.Errorf(`ent: validator failed for field "UsageLog.model": %w`, err)}
 		}
 	}
+	if v, ok := _u.mutation.UserAgent(); ok {
+		if err := usagelog.UserAgentValidator(v); err != nil {
+			return &ValidationError{Name: "user_agent", err: fmt.Errorf(`ent: validator failed for field "UsageLog.user_agent": %w`, err)}
+		}
+	}
 	if v, ok := _u.mutation.ImageSize(); ok {
 		if err := usagelog.ImageSizeValidator(v); err != nil {
 			return &ValidationError{Name: "image_size", err: fmt.Errorf(`ent: validator failed for field "UsageLog.image_size": %w`, err)}
@@ -1743,6 +1799,12 @@ func (_u *UsageLogUpdateOne) sqlSave(ctx context.Context) (_node *UsageLog, err
 	if _u.mutation.FirstTokenMsCleared() {
 		_spec.ClearField(usagelog.FieldFirstTokenMs, field.TypeInt)
 	}
+	if value, ok := _u.mutation.UserAgent(); ok {
+		_spec.SetField(usagelog.FieldUserAgent, field.TypeString, value)
+	}
+	if _u.mutation.UserAgentCleared() {
+		_spec.ClearField(usagelog.FieldUserAgent, field.TypeString)
+	}
 	if value, ok := _u.mutation.ImageCount(); ok {
 		_spec.SetField(usagelog.FieldImageCount, field.TypeInt, value)
 	}