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">

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">

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>
         ) : (