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/models/user.py

@@ -33,6 +33,7 @@ class User(Base):
     tasks = relationship("Task", back_populates="user", cascade="all, delete-orphan")
     sessions = relationship("Session", back_populates="user", cascade="all, delete-orphan")
     audit_logs = relationship("AuditLog", back_populates="user")
+    translation_logs = relationship("TranslationLog", back_populates="user", cascade="all, delete-orphan")
 
     def __repr__(self):
         return f"<User(id={self.id}, email='{self.email}', display_name='{self.display_name}')>"