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/copilot_service.py

@@ -43,7 +43,7 @@ def _parse_ai_response(raw_content: str) -> dict[str, Any]:
     # Strip markdown code fences if present
     if text.startswith("```"):
         lines = text.split("\n")
-        lines = [l for l in lines if not l.strip().startswith("```")]
+        lines = [line for line in lines if not line.strip().startswith("```")]
         text = "\n".join(lines).strip()
 
     try: