feat: add translation billing stats and remove Export/Settings pages

- Add TranslationLog model to track translation API usage per task
- Integrate Dify API actual price (total_price) into translation stats
- Display translation statistics in admin dashboard with per-task costs
- Remove unused Export and Settings pages to simplify frontend
- Add GET /api/v2/admin/translation-stats endpoint

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/routers/admin.py

@@ -186,3 +186,34 @@ async def get_user_activity_summary(
             status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
             detail=f"Failed to get user activity summary: {str(e)}"
         )
+
+
+@router.get("/translation-stats", summary="Get translation statistics")
+async def get_translation_stats(
+    db: Session = Depends(get_db),
+    admin_user: User = Depends(get_current_admin_user)
+):
+    """
+    Get translation usage statistics for billing and monitoring.
+
+    Returns:
+    - total_translations: Total number of translation jobs
+    - total_tokens: Sum of all tokens used
+    - total_characters: Sum of all characters translated
+    - estimated_cost: Estimated cost based on token pricing
+    - by_language: Breakdown by target language
+    - recent_translations: List of recent translation activities
+    - last_30_days: Statistics for the last 30 days
+
+    Requires admin privileges.
+    """
+    try:
+        stats = admin_service.get_translation_statistics(db)
+        return stats
+
+    except Exception as e:
+        logger.exception("Failed to get translation statistics")
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail=f"Failed to get translation statistics: {str(e)}"
+        )