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/FilePreview.tsx

@@ -1,5 +1,6 @@
 'use client';
 
+import Image from 'next/image';
 import { Modal } from '@/components/ui/Modal';
 import { isImageMime, formatFileSize, type FileResponse } from '@/lib/files';
 
@@ -22,11 +23,14 @@ export function FilePreview({ file, open, onClose }: FilePreviewProps) {
         {/* Preview content */}
         {isImg ? (
           <div className="flex justify-center">
-            <img
+            <Image
               src={downloadUrl}
               alt={file.original_filename}
+              width={800}
+              height={600}
               className="max-w-full max-h-[60vh] rounded-lg"
               data-testid="preview-image"
+              unoptimized
             />
           </div>
         ) : (