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/utils/contract_pdf.py

@@ -10,7 +10,6 @@ from datetime import date
 from decimal import Decimal
 from typing import Any
 
-from app.config import settings
 
 
 # HTML template for the sales contract

backend/app/utils/datev.py

@@ -6,7 +6,6 @@ Datum, Konto, Gegenkonto, Betrag, Belegfeld, Buchungstext
 
 import csv
 import io
-from datetime import date
 from decimal import Decimal
 from typing import Any
 

backend/app/utils/openrouter.py

@@ -84,7 +84,7 @@ def _parse_response(raw_content: str) -> dict[str, Any]:
     if text.startswith("```"):
         lines = text.split("\n")
         # Remove first line (```json or ```) and last line (```)
-        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: