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/services/admin_service.py

@@ -13,6 +13,7 @@ from app.models.user import User
 from app.models.task import Task, TaskStatus
 from app.models.session import Session as UserSession
 from app.models.audit_log import AuditLog
+from app.models.translation_log import TranslationLog
 from app.core.config import settings
 
 logger = logging.getLogger(__name__)
@@ -209,6 +210,87 @@ class AdminService:
 
         return top_users
 
+    def get_translation_statistics(self, db: Session) -> dict:
+        """
+        Get translation usage statistics for admin dashboard.
+
+        Args:
+            db: Database session
+
+        Returns:
+            Dictionary with translation stats including total tokens, costs, and breakdowns
+        """
+        # Total translation count
+        total_translations = db.query(TranslationLog).count()
+
+        # Sum of tokens
+        token_stats = db.query(
+            func.sum(TranslationLog.total_tokens).label("total_tokens"),
+            func.sum(TranslationLog.input_tokens).label("total_input_tokens"),
+            func.sum(TranslationLog.output_tokens).label("total_output_tokens"),
+            func.sum(TranslationLog.total_characters).label("total_characters"),
+            func.sum(TranslationLog.estimated_cost).label("total_cost")
+        ).first()
+
+        # Breakdown by target language
+        by_language = db.query(
+            TranslationLog.target_lang,
+            func.count(TranslationLog.id).label("count"),
+            func.sum(TranslationLog.total_tokens).label("tokens"),
+            func.sum(TranslationLog.total_characters).label("characters")
+        ).group_by(TranslationLog.target_lang).all()
+
+        language_breakdown = [
+            {
+                "language": lang,
+                "count": count,
+                "tokens": tokens or 0,
+                "characters": chars or 0
+            }
+            for lang, count, tokens, chars in by_language
+        ]
+
+        # Recent translations (last 20)
+        recent = db.query(TranslationLog).order_by(
+            TranslationLog.created_at.desc()
+        ).limit(20).all()
+
+        recent_translations = [
+            {
+                "id": log.id,
+                "task_id": log.task_id,
+                "target_lang": log.target_lang,
+                "total_tokens": log.total_tokens,
+                "total_characters": log.total_characters,
+                "processing_time_seconds": log.processing_time_seconds,
+                "estimated_cost": log.estimated_cost,
+                "created_at": log.created_at.isoformat() if log.created_at else None
+            }
+            for log in recent
+        ]
+
+        # Stats for last 30 days
+        date_30_days_ago = datetime.utcnow() - timedelta(days=30)
+        recent_stats = db.query(
+            func.count(TranslationLog.id).label("count"),
+            func.sum(TranslationLog.total_tokens).label("tokens")
+        ).filter(TranslationLog.created_at >= date_30_days_ago).first()
+
+        return {
+            "total_translations": total_translations,
+            "total_tokens": token_stats.total_tokens or 0,
+            "total_input_tokens": token_stats.total_input_tokens or 0,
+            "total_output_tokens": token_stats.total_output_tokens or 0,
+            "total_characters": token_stats.total_characters or 0,
+            "estimated_cost": token_stats.total_cost or 0.0,
+            "by_language": language_breakdown,
+            "recent_translations": recent_translations,
+            "last_30_days": {
+                "count": recent_stats.count or 0,
+                "tokens": recent_stats.tokens or 0
+            }
+        }
+
 
 # Singleton instance
 admin_service = AdminService()