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/FileList.tsx

@@ -1,6 +1,7 @@
 'use client';
 
 import { useState, useEffect, useCallback } from 'react';
+import Image from 'next/image';
 import { Button } from '@/components/ui/Button';
 import { Modal } from '@/components/ui/Modal';
 import {
@@ -124,13 +125,15 @@ export function FileList({ vehicleId, onFileDeleted }: FileListProps) {
               data-testid={`file-item-${file.id}`}
             >
               {/* Thumbnail or file icon */}
-              <div className="aspect-square bg-background flex items-center justify-center">
+              <div className="aspect-square bg-background flex items-center justify-center relative">
                 {isImg && thumbUrl ? (
-                  <img
+                  <Image
                     src={thumbUrl}
                     alt={file.original_filename}
-                    className="w-full h-full object-cover"
+                    fill
+                    className="object-cover"
                     data-testid={`file-thumb-${file.id}`}
+                    unoptimized
                   />
                 ) : (
                   <div className="flex flex-col items-center gap-2 text-text-muted">