feat(usage): add User-Agent column to usage logs

- Add user_agent field to UsageLog DTO and mapper
- Display User-Agent column in admin and user usage tables
- Add formatUserAgent helper to show friendly client names
- Include user_agent in Excel export
- Remove request_id column from admin usage table

backend/internal/handler/dto/mappers.go

@@ -266,6 +266,7 @@ func UsageLogFromService(l *service.UsageLog) *UsageLog {
 		FirstTokenMs:          l.FirstTokenMs,
 		ImageCount:            l.ImageCount,
 		ImageSize:             l.ImageSize,
+		UserAgent:             l.UserAgent,
 		CreatedAt:             l.CreatedAt,
 		User:                  UserFromServiceShallow(l.User),
 		APIKey:                APIKeyFromService(l.APIKey),

backend/internal/handler/dto/types.go

@@ -180,6 +180,9 @@ type UsageLog struct {
 	ImageCount int     `json:"image_count"`
 	ImageSize  *string `json:"image_size"`
 
+	// User-Agent
+	UserAgent *string `json:"user_agent"`
+
 	CreatedAt time.Time `json:"created_at"`
 
 	User         *User             `json:"user,omitempty"`