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/mutation.go

@@ -8107,6 +8107,7 @@ type UsageLogMutation struct {
 	addduration_ms              *int
 	first_token_ms              *int
 	addfirst_token_ms           *int
+	user_agent                  *string
 	image_count                 *int
 	addimage_count              *int
 	image_size                  *string
@@ -9463,6 +9464,55 @@ func (m *UsageLogMutation) ResetFirstTokenMs() {
 	delete(m.clearedFields, usagelog.FieldFirstTokenMs)
 }
 
+// SetUserAgent sets the "user_agent" field.
+func (m *UsageLogMutation) SetUserAgent(s string) {
+	m.user_agent = &s
+}
+
+// UserAgent returns the value of the "user_agent" field in the mutation.
+func (m *UsageLogMutation) UserAgent() (r string, exists bool) {
+	v := m.user_agent
+	if v == nil {
+		return
+	}
+	return *v, true
+}
+
+// OldUserAgent returns the old "user_agent" field's value of the UsageLog entity.
+// If the UsageLog object wasn't provided to the builder, the object is fetched from the database.
+// An error is returned if the mutation operation is not UpdateOne, or the database query fails.
+func (m *UsageLogMutation) OldUserAgent(ctx context.Context) (v *string, err error) {
+	if !m.op.Is(OpUpdateOne) {
+		return v, errors.New("OldUserAgent is only allowed on UpdateOne operations")
+	}
+	if m.id == nil || m.oldValue == nil {
+		return v, errors.New("OldUserAgent requires an ID field in the mutation")
+	}
+	oldValue, err := m.oldValue(ctx)
+	if err != nil {
+		return v, fmt.Errorf("querying old value for OldUserAgent: %w", err)
+	}
+	return oldValue.UserAgent, nil
+}
+
+// ClearUserAgent clears the value of the "user_agent" field.
+func (m *UsageLogMutation) ClearUserAgent() {
+	m.user_agent = nil
+	m.clearedFields[usagelog.FieldUserAgent] = struct{}{}
+}
+
+// UserAgentCleared returns if the "user_agent" field was cleared in this mutation.
+func (m *UsageLogMutation) UserAgentCleared() bool {
+	_, ok := m.clearedFields[usagelog.FieldUserAgent]
+	return ok
+}
+
+// ResetUserAgent resets all changes to the "user_agent" field.
+func (m *UsageLogMutation) ResetUserAgent() {
+	m.user_agent = nil
+	delete(m.clearedFields, usagelog.FieldUserAgent)
+}
+
 // SetImageCount sets the "image_count" field.
 func (m *UsageLogMutation) SetImageCount(i int) {
 	m.image_count = &i
@@ -9773,7 +9823,7 @@ func (m *UsageLogMutation) Type() string {
 // order to get all numeric fields that were incremented/decremented, call
 // AddedFields().
 func (m *UsageLogMutation) Fields() []string {
-	fields := make([]string, 0, 27)
+	fields := make([]string, 0, 28)
 	if m.user != nil {
 		fields = append(fields, usagelog.FieldUserID)
 	}
@@ -9846,6 +9896,9 @@ func (m *UsageLogMutation) Fields() []string {
 	if m.first_token_ms != nil {
 		fields = append(fields, usagelog.FieldFirstTokenMs)
 	}
+	if m.user_agent != nil {
+		fields = append(fields, usagelog.FieldUserAgent)
+	}
 	if m.image_count != nil {
 		fields = append(fields, usagelog.FieldImageCount)
 	}
@@ -9911,6 +9964,8 @@ func (m *UsageLogMutation) Field(name string) (ent.Value, bool) {
 		return m.DurationMs()
 	case usagelog.FieldFirstTokenMs:
 		return m.FirstTokenMs()
+	case usagelog.FieldUserAgent:
+		return m.UserAgent()
 	case usagelog.FieldImageCount:
 		return m.ImageCount()
 	case usagelog.FieldImageSize:
@@ -9974,6 +10029,8 @@ func (m *UsageLogMutation) OldField(ctx context.Context, name string) (ent.Value
 		return m.OldDurationMs(ctx)
 	case usagelog.FieldFirstTokenMs:
 		return m.OldFirstTokenMs(ctx)
+	case usagelog.FieldUserAgent:
+		return m.OldUserAgent(ctx)
 	case usagelog.FieldImageCount:
 		return m.OldImageCount(ctx)
 	case usagelog.FieldImageSize:
@@ -10157,6 +10214,13 @@ func (m *UsageLogMutation) SetField(name string, value ent.Value) error {
 		}
 		m.SetFirstTokenMs(v)
 		return nil
+	case usagelog.FieldUserAgent:
+		v, ok := value.(string)
+		if !ok {
+			return fmt.Errorf("unexpected type %T for field %s", value, name)
+		}
+		m.SetUserAgent(v)
+		return nil
 	case usagelog.FieldImageCount:
 		v, ok := value.(int)
 		if !ok {
@@ -10427,6 +10491,9 @@ func (m *UsageLogMutation) ClearedFields() []string {
 	if m.FieldCleared(usagelog.FieldFirstTokenMs) {
 		fields = append(fields, usagelog.FieldFirstTokenMs)
 	}
+	if m.FieldCleared(usagelog.FieldUserAgent) {
+		fields = append(fields, usagelog.FieldUserAgent)
+	}
 	if m.FieldCleared(usagelog.FieldImageSize) {
 		fields = append(fields, usagelog.FieldImageSize)
 	}
@@ -10456,6 +10523,9 @@ func (m *UsageLogMutation) ClearField(name string) error {
 	case usagelog.FieldFirstTokenMs:
 		m.ClearFirstTokenMs()
 		return nil
+	case usagelog.FieldUserAgent:
+		m.ClearUserAgent()
+		return nil
 	case usagelog.FieldImageSize:
 		m.ClearImageSize()
 		return nil
@@ -10539,6 +10609,9 @@ func (m *UsageLogMutation) ResetField(name string) error {
 	case usagelog.FieldFirstTokenMs:
 		m.ResetFirstTokenMs()
 		return nil
+	case usagelog.FieldUserAgent:
+		m.ResetUserAgent()
+		return nil
 	case usagelog.FieldImageCount:
 		m.ResetImageCount()
 		return nil