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.go

@@ -70,6 +70,8 @@ type UsageLog struct {
 	DurationMs *int `json:"duration_ms,omitempty"`
 	// FirstTokenMs holds the value of the "first_token_ms" field.
 	FirstTokenMs *int `json:"first_token_ms,omitempty"`
+	// UserAgent holds the value of the "user_agent" field.
+	UserAgent *string `json:"user_agent,omitempty"`
 	// ImageCount holds the value of the "image_count" field.
 	ImageCount int `json:"image_count,omitempty"`
 	// ImageSize holds the value of the "image_size" field.
@@ -165,7 +167,7 @@ func (*UsageLog) scanValues(columns []string) ([]any, error) {
 			values[i] = new(sql.NullFloat64)
 		case usagelog.FieldID, usagelog.FieldUserID, usagelog.FieldAPIKeyID, usagelog.FieldAccountID, usagelog.FieldGroupID, usagelog.FieldSubscriptionID, usagelog.FieldInputTokens, usagelog.FieldOutputTokens, usagelog.FieldCacheCreationTokens, usagelog.FieldCacheReadTokens, usagelog.FieldCacheCreation5mTokens, usagelog.FieldCacheCreation1hTokens, usagelog.FieldBillingType, usagelog.FieldDurationMs, usagelog.FieldFirstTokenMs, usagelog.FieldImageCount:
 			values[i] = new(sql.NullInt64)
-		case usagelog.FieldRequestID, usagelog.FieldModel, usagelog.FieldImageSize:
+		case usagelog.FieldRequestID, usagelog.FieldModel, usagelog.FieldUserAgent, usagelog.FieldImageSize:
 			values[i] = new(sql.NullString)
 		case usagelog.FieldCreatedAt:
 			values[i] = new(sql.NullTime)
@@ -338,6 +340,13 @@ func (_m *UsageLog) assignValues(columns []string, values []any) error {
 				_m.FirstTokenMs = new(int)
 				*_m.FirstTokenMs = int(value.Int64)
 			}
+		case usagelog.FieldUserAgent:
+			if value, ok := values[i].(*sql.NullString); !ok {
+				return fmt.Errorf("unexpected type %T for field user_agent", values[i])
+			} else if value.Valid {
+				_m.UserAgent = new(string)
+				*_m.UserAgent = value.String
+			}
 		case usagelog.FieldImageCount:
 			if value, ok := values[i].(*sql.NullInt64); !ok {
 				return fmt.Errorf("unexpected type %T for field image_count", values[i])
@@ -498,6 +507,11 @@ func (_m *UsageLog) String() string {
 		builder.WriteString(fmt.Sprintf("%v", *v))
 	}
 	builder.WriteString(", ")
+	if v := _m.UserAgent; v != nil {
+		builder.WriteString("user_agent=")
+		builder.WriteString(*v)
+	}
+	builder.WriteString(", ")
 	builder.WriteString("image_count=")
 	builder.WriteString(fmt.Sprintf("%v", _m.ImageCount))
 	builder.WriteString(", ")