feat: add product image upload to admin panel

- Add storage.ts utility (saveImage, deleteImageFile) for local disk operations
- Add POST /api/admin/products/[id]/images: validates MIME type and 5MB limit, saves file, creates MediaAsset record
- Add DELETE /api/admin/products/[id]/images?imageId=: removes file and DB record
- Add Images section to product edit form (hidden for new products until saved)
- Display images in square aspect-ratio grid matching storefront display
- Support multi-file upload; hover to reveal delete button

app/src/lib/storage.ts

@@ -0,0 +1,22 @@
+import { mkdir, writeFile, unlink } from 'fs/promises'
+import path from 'path'
+
+const UPLOAD_DIR = path.join(process.cwd(), 'public', 'uploads')
+
+export async function saveImage(
+  productId: string,
+  buffer: Buffer,
+  originalFilename: string
+): Promise<string> {
+  const dir = path.join(UPLOAD_DIR, productId)
+  await mkdir(dir, { recursive: true })
+  const safeName = originalFilename.replace(/[^a-z0-9._-]/gi, '_').toLowerCase()
+  const filename = `${Date.now()}-${safeName}`
+  await writeFile(path.join(dir, filename), buffer)
+  return `/uploads/${productId}/${filename}`
+}
+
+export async function deleteImageFile(url: string): Promise<void> {
+  const filePath = path.join(process.cwd(), 'public', url)
+  await unlink(filePath)
+}