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/routers/datev.py

@@ -2,7 +2,7 @@
 
 import uuid
 
-from fastapi import APIRouter, Depends, HTTPException, Query, status
+from fastapi import APIRouter, Depends, HTTPException, status
 from fastapi.responses import Response
 from sqlalchemy.ext.asyncio import AsyncSession
 

backend/app/routers/users.py

@@ -2,11 +2,11 @@
 
 import uuid
 
-from fastapi import APIRouter, Depends, HTTPException, Query, status
+from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
-from app.dependencies import get_current_user, get_pagination, require_role
+from app.dependencies import get_pagination, require_role
 from app.models.user import User
 from app.schemas.user import (
     UserCreate,
@@ -17,7 +17,6 @@ from app.schemas.user import (
 from app.services.auth_service import (
     create_user as svc_create_user,
     deactivate_user as svc_deactivate_user,
-    get_user_by_id,
     list_users as svc_list_users,
     update_user as svc_update_user,
 )

backend/app/routers/vehicles.py

@@ -8,7 +8,6 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.database import get_db
 from app.dependencies import get_current_user, get_pagination
 from app.models.user import User
-from app.models.vehicle import Vehicle
 from app.schemas.vehicle import (
     MobileDePushResponse,
     MobileDeStatusResponse,