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

frontend/src/types/index.ts

@@ -576,6 +576,9 @@ export interface UsageLog {
   image_count: number
   image_size: string | null
 
+  // User-Agent
+  user_agent: string | null
+
   created_at: string
 
   user?: User