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)}"
+        )

backend/app/routers/translate.py

@@ -39,7 +39,8 @@ def run_translation_task(
     task_id: str,
     task_db_id: int,
     target_lang: str,
-    source_lang: str = "auto"
+    source_lang: str = "auto",
+    user_id: int = None
 ):
     """
     Background task to run document translation.
@@ -49,10 +50,12 @@ def run_translation_task(
         task_db_id: Task database ID (for verification)
         target_lang: Target language code
         source_lang: Source language code ('auto' for detection)
+        user_id: User ID for logging translation statistics
     """
     from app.core.database import SessionLocal
     from app.services.translation_service import get_translation_service
     from app.schemas.translation import TranslationJobState, TranslationProgress
+    from app.models.translation_log import TranslationLog
 
     db = SessionLocal()
     translation_service = get_translation_service()
@@ -132,6 +135,44 @@ def run_translation_task(
                 result_file_path=str(output_path) if output_path else None
             ))
             logger.info(f"Translation completed for task {task_id}")
+
+            # Log translation statistics to database
+            if user_id and output_path:
+                try:
+                    with open(output_path, 'r', encoding='utf-8') as f:
+                        translation_result = json.load(f)
+
+                    stats = translation_result.get('statistics', {})
+                    total_tokens = stats.get('total_tokens', 0)
+
+                    # Use actual price from Dify API if available, otherwise calculate estimated cost
+                    actual_price = stats.get('total_price', 0.0)
+                    if actual_price > 0:
+                        estimated_cost = actual_price
+                    else:
+                        # Fallback: Calculate estimated cost based on token pricing
+                        estimated_cost = (total_tokens / 1_000_000) * settings.translation_cost_per_million_tokens
+
+                    translation_log = TranslationLog(
+                        user_id=user_id,
+                        task_id=task_id,
+                        target_lang=target_lang,
+                        source_lang=source_lang,
+                        total_tokens=total_tokens,
+                        input_tokens=0,  # Dify doesn't provide separate input/output tokens
+                        output_tokens=0,
+                        total_elements=stats.get('total_elements', 0),
+                        translated_elements=stats.get('translated_elements', 0),
+                        total_characters=stats.get('total_characters', 0),
+                        processing_time_seconds=stats.get('processing_time_seconds', 0.0),
+                        provider=translation_result.get('provider', 'dify'),
+                        estimated_cost=estimated_cost
+                    )
+                    db.add(translation_log)
+                    db.commit()
+                    logger.info(f"Logged translation stats for task {task_id}: {total_tokens} tokens, ${estimated_cost:.6f}")
+                except Exception as log_error:
+                    logger.error(f"Failed to log translation stats: {log_error}")
         else:
             translation_service.set_job_state(task_id, TranslationJobState(
                 task_id=task_id,
@@ -255,7 +296,8 @@ async def start_translation(
         task_id=task_id,
         task_db_id=task.id,
         target_lang=target_lang,
-        source_lang=request.source_lang
+        source_lang=request.source_lang,
+        user_id=current_user.id
     )
 
     logger.info(f"Started translation job for task {task_id}, target_lang={target_lang}")