feat(images): add sha256-deduplicated image downloader

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/server/images/image-downloader.ts

@@ -0,0 +1,40 @@
+import { createHash } from 'node:crypto';
+import { existsSync } from 'node:fs';
+import { mkdir, writeFile } from 'node:fs/promises';
+import { join } from 'node:path';
+import { fetchBuffer } from '../http';
+
+const EXT_BY_CONTENT_TYPE: Record<string, string> = {
+  'image/jpeg': '.jpg',
+  'image/jpg': '.jpg',
+  'image/png': '.png',
+  'image/webp': '.webp',
+  'image/gif': '.gif',
+  'image/avif': '.avif'
+};
+
+function extensionFor(contentType: string | null): string {
+  if (!contentType) return '.bin';
+  const base = contentType.split(';')[0].trim().toLowerCase();
+  return EXT_BY_CONTENT_TYPE[base] ?? '.bin';
+}
+
+export async function downloadImage(
+  url: string,
+  targetDir: string
+): Promise<string | null> {
+  try {
+    const { data, contentType } = await fetchBuffer(url, { maxBytes: 10 * 1024 * 1024 });
+    const hash = createHash('sha256').update(data).digest('hex');
+    const ext = extensionFor(contentType);
+    const filename = `${hash}${ext}`;
+    const target = join(targetDir, filename);
+    if (!existsSync(target)) {
+      await mkdir(targetDir, { recursive: true });
+      await writeFile(target, data);
+    }
+    return filename;
+  } catch {
+    return null;
+  }
+}