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/retouch/BeforeAfterSlider.tsx

@@ -1,6 +1,7 @@
 'use client';
 
 import { useState, useCallback, useRef, useEffect } from 'react';
+import Image from 'next/image';
 
 interface BeforeAfterSliderProps {
   beforeSrc: string;
@@ -78,12 +79,14 @@ export function BeforeAfterSlider({
       onClick={handleClick}
     >
       {/* After image (full, background) */}
-      <img
+      <Image
         src={afterSrc}
         alt={afterLabel}
-        className="absolute inset-0 w-full h-full object-cover"
+        fill
+        className="object-cover"
         data-testid="after-image"
         draggable={false}
+        unoptimized
       />
 
       {/* Before image (clipped to left of slider) */}
@@ -91,13 +94,15 @@ export function BeforeAfterSlider({
         className="absolute inset-0 overflow-hidden"
         style={{ width: `${sliderPos}%` }}
       >
-        <img
+        <Image
           src={beforeSrc}
           alt={beforeLabel}
-          className="absolute inset-0 h-full object-cover"
+          fill
+          className="object-cover"
           style={{ width: `${containerRef.current?.clientWidth || 100}%` }}
           data-testid="before-image"
           draggable={false}
+          unoptimized
         />
       </div>