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:

backend/app/services/datev_service.py

@@ -6,7 +6,6 @@ import os
 import uuid
 from datetime import date
 from decimal import Decimal
-from typing import Any
 
 from sqlalchemy import and_, func, select
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -14,7 +13,7 @@ from sqlalchemy.orm import selectinload
 
 from app.models.datev_export import DATEVExport
 from app.models.sale import Sale
-from app.utils.datev import generate_datev_csv, validate_datev_csv
+from app.utils.datev import generate_datev_csv
 
 
 async def create_export(

backend/app/services/file_service.py

@@ -8,7 +8,6 @@ Enforces MAX_FILE_SIZE_MB from config (default 50MB, but endpoint enforces 20MB)
 from __future__ import annotations
 
 import logging
-import os
 import uuid
 from pathlib import Path
 from typing import Optional

backend/app/services/mobilede_service.py

@@ -8,10 +8,9 @@ from __future__ import annotations
 
 import uuid
 from datetime import datetime, timezone
-from typing import Any
 
 import httpx
-from sqlalchemy import and_, select
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.config import settings

backend/app/services/ocr_service.py

@@ -5,8 +5,6 @@ from __future__ import annotations
 import logging
 import os
 import uuid
-from datetime import datetime, timezone
-from typing import Any
 
 from sqlalchemy import and_, func, select
 from sqlalchemy.ext.asyncio import AsyncSession

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
 

backend/app/services/sale_service.py

@@ -3,8 +3,7 @@
 from __future__ import annotations
 
 import uuid
-from datetime import date, datetime, timezone
-from decimal import Decimal
+from datetime import date
 from typing import Any
 
 from sqlalchemy import and_, func, select
@@ -26,7 +25,6 @@ async def create_sale(db: AsyncSession, data: dict[str, Any]) -> Sale:
     Raises ValueError if vehicle or buyer contact not found.
     """
     vehicle_id = data["vehicle_id"]
-    buyer_contact_id = data["buyer_contact_id"]
 
     # Verify vehicle exists
     vehicle = await db.get(Vehicle, vehicle_id)

backend/app/services/vehicle_service.py

@@ -8,7 +8,6 @@ from typing import Any
 
 from sqlalchemy import and_, func, or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy.orm import selectinload
 
 from app.models.vehicle import Vehicle