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/file.py

@@ -2,6 +2,7 @@
 
 import uuid
 from datetime import datetime
+from typing import TYPE_CHECKING
 
 from sqlalchemy import DateTime, ForeignKey, Integer, String, func
 from sqlalchemy.dialects.postgresql import UUID
@@ -9,6 +10,9 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database import Base
 
+if TYPE_CHECKING:
+    from app.models.vehicle import Vehicle
+
 
 class File(Base):
     """File attachment entity linked to a vehicle."""