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

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+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;
+  }
+}

tests/integration/image-downloader.test.ts

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+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { createServer, type Server } from 'node:http';
+import type { AddressInfo } from 'node:net';
+import { mkdtemp, readFile, rm } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { downloadImage } from '../../src/lib/server/images/image-downloader';
+
+// 1×1 PNG (transparent)
+const PNG_BYTES = Buffer.from(
+  '89504e470d0a1a0a0000000d49484452000000010000000108060000001f15c489' +
+    '0000000d49444154789c6300010000000500010d0a2db40000000049454e44ae426082',
+  'hex'
+);
+
+let server: Server;
+let baseUrl: string;
+let dir: string;
+
+beforeEach(async () => {
+  server = createServer();
+  await new Promise<void>((r) => server.listen(0, '127.0.0.1', r));
+  const addr = server.address() as AddressInfo;
+  baseUrl = `http://127.0.0.1:${addr.port}`;
+  dir = await mkdtemp(join(tmpdir(), 'kochwas-img-'));
+});
+
+afterEach(async () => {
+  await new Promise<void>((r) => server.close(() => r()));
+  await rm(dir, { recursive: true, force: true });
+});
+
+describe('downloadImage', () => {
+  it('downloads and stores a PNG with sha256 filename', async () => {
+    server.on('request', (_req, res) => {
+      res.writeHead(200, { 'content-type': 'image/png' });
+      res.end(PNG_BYTES);
+    });
+    const fname = await downloadImage(`${baseUrl}/img.png`, dir);
+    expect(fname).not.toBeNull();
+    expect(fname!.endsWith('.png')).toBe(true);
+    const saved = await readFile(join(dir, fname!));
+    expect(saved.equals(PNG_BYTES)).toBe(true);
+  });
+
+  it('is idempotent — returns same filename, does not rewrite', async () => {
+    server.on('request', (_req, res) => {
+      res.writeHead(200, { 'content-type': 'image/png' });
+      res.end(PNG_BYTES);
+    });
+    const first = await downloadImage(`${baseUrl}/img.png`, dir);
+    const second = await downloadImage(`${baseUrl}/img.png`, dir);
+    expect(second).toBe(first);
+  });
+
+  it('returns null on 404', async () => {
+    server.on('request', (_req, res) => {
+      res.writeHead(404);
+      res.end();
+    });
+    const fname = await downloadImage(`${baseUrl}/missing.png`, dir);
+    expect(fname).toBeNull();
+  });
+});