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/models/sale.py

@@ -4,6 +4,7 @@ import enum
 import uuid
 from datetime import date, datetime
 from decimal import Decimal
+from typing import TYPE_CHECKING
 
 from sqlalchemy import (
     Boolean,
@@ -20,6 +21,10 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database import Base
 
+if TYPE_CHECKING:
+    from app.models.contact import Contact
+    from app.models.vehicle import Vehicle
+
 
 class SaleStatus(str, enum.Enum):
     draft = "draft"