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/price_compare_service.py

@@ -5,7 +5,6 @@ from __future__ import annotations
 import logging
 import random
 import uuid
-from typing import Any
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -83,7 +82,7 @@ async def compare_prices(
 
     # Calculate average price
     if listings:
-        average_price = round(sum(l.price for l in listings) / len(listings), 2)
+        average_price = round(sum(listing.price for listing in listings) / len(listings), 2)
     else:
         average_price = None