fix: resolve all ruff and ESLint lint errors

Backend (ruff):
- F821: Add TYPE_CHECKING imports for Vehicle, Contact, File in models
  (file.py, retouch.py, sale.py, vehicle.py)
- E741: Rename ambiguous variable  to /
  (copilot_service.py, price_compare_service.py, openrouter.py)
- F841: Remove unused variable  in sale_service.py

Frontend (ESLint):
- react/no-unescaped-entities: Escape quotes in ContractPreview.tsx
- @next/next/no-img-element: Replace <img> with <Image> from next/image
  (FileGallery.tsx, FileList.tsx, FilePreview.tsx, BeforeAfterSlider.tsx)

Tests: 392 backend passed, 112 frontend passed, next build successful

backend/app/services/sale_service.py

@@ -3,8 +3,7 @@
 from __future__ import annotations
 
 import uuid
-from datetime import date, datetime, timezone
-from decimal import Decimal
+from datetime import date
 from typing import Any
 
 from sqlalchemy import and_, func, select
@@ -26,7 +25,6 @@ async def create_sale(db: AsyncSession, data: dict[str, Any]) -> Sale:
     Raises ValueError if vehicle or buyer contact not found.
     """
     vehicle_id = data["vehicle_id"]
-    buyer_contact_id = data["buyer_contact_id"]
 
     # Verify vehicle exists
     vehicle = await db.get(Vehicle, vehicle_id)