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."""

backend/app/models/retouch.py

@@ -3,6 +3,7 @@
 import enum
 import uuid
 from datetime import datetime
+from typing import TYPE_CHECKING
 
 from sqlalchemy import DateTime, ForeignKey, String, Text, func
 from sqlalchemy.dialects.postgresql import UUID
@@ -10,6 +11,9 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database import Base
 
+if TYPE_CHECKING:
+    from app.models.vehicle import Vehicle
+
 
 class RetouchStatus(str, enum.Enum):
     pending = "pending"

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"

backend/app/models/user.py

@@ -2,7 +2,7 @@
 
 import enum
 import uuid
-from datetime import datetime, timezone
+from datetime import datetime
 
 from sqlalchemy import Boolean, DateTime, Enum, String, func
 from sqlalchemy.dialects.postgresql import UUID

backend/app/models/vehicle.py

@@ -4,12 +4,12 @@ import enum
 import uuid
 from datetime import date, datetime
 from decimal import Decimal
+from typing import TYPE_CHECKING
 
 from sqlalchemy import (
     CheckConstraint,
     Date,
     DateTime,
-    Enum,
     ForeignKey,
     Integer,
     Numeric,
@@ -22,6 +22,9 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database import Base
 
+if TYPE_CHECKING:
+    from app.models.file import File
+
 
 class VehicleCondition(str, enum.Enum):
     new = "new"

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,

backend/app/schemas/contact.py

@@ -4,7 +4,7 @@ import uuid
 from datetime import datetime
 from typing import Literal, Optional
 
-from pydantic import BaseModel, ConfigDict, EmailStr, Field, field_validator
+from pydantic import BaseModel, ConfigDict, Field, field_validator
 
 from app.utils.ust_validation import validate_vat_id
 

backend/app/schemas/vehicle.py

@@ -5,7 +5,7 @@ from datetime import date, datetime
 from decimal import Decimal
 from typing import Literal, Optional
 
-from pydantic import BaseModel, ConfigDict, Field, field_validator, model_validator
+from pydantic import BaseModel, ConfigDict, Field, model_validator
 
 
 VEHICLE_TYPES = Literal["lkw", "pkw", "baumaschine", "stapler", "transporter"]

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
 

backend/app/tasks/ocr_processing.py

@@ -9,7 +9,6 @@ from __future__ import annotations
 import logging
 import uuid
 
-from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import async_session_factory
 from app.services.ocr_service import process_ocr

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: