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

frontend/components/files/FileGallery.tsx

@@ -7,6 +7,7 @@ import {
   getThumbnailUrl,
   type FileResponse,
 } from '@/lib/files';
+import Image from 'next/image';
 import { FilePreview } from './FilePreview';
 
 interface FileGalleryProps {
@@ -76,10 +77,12 @@ export function FileGallery({ vehicleId }: FileGalleryProps) {
               data-testid={`gallery-item-${image.id}`}
             >
               {thumbUrl && (
-                <img
+                <Image
                   src={thumbUrl}
                   alt={image.original_filename}
-                  className="w-full h-full object-cover transition-transform group-hover:scale-105"
+                  fill
+                  className="object-cover transition-transform group-hover:scale-105"
+                  unoptimized
                 />
               )}
               <div className="absolute inset-0 bg-black/0 group-hover:bg-black/30 transition-colors flex items-end p-2">