feat(backup): add ZIP export endpoint (DB + images)

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

src/lib/server/backup/export.ts

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+import archiver from 'archiver';
+import { createReadStream, existsSync, readdirSync, statSync } from 'node:fs';
+import { join } from 'node:path';
+import { Readable } from 'node:stream';
+
+type Options = {
+  dbPath: string;
+  imagesDir: string;
+};
+
+export function createBackupStream({ dbPath, imagesDir }: Options): Readable {
+  const archive = archiver('zip', { zlib: { level: 6 } });
+
+  // DB file — include .db plus WAL if present (best-effort)
+  if (existsSync(dbPath)) {
+    archive.file(dbPath, { name: 'kochwas.db' });
+    const wal = `${dbPath}-wal`;
+    if (existsSync(wal)) archive.file(wal, { name: 'kochwas.db-wal' });
+    const shm = `${dbPath}-shm`;
+    if (existsSync(shm)) archive.file(shm, { name: 'kochwas.db-shm' });
+  }
+
+  // Images
+  if (existsSync(imagesDir) && statSync(imagesDir).isDirectory()) {
+    for (const name of readdirSync(imagesDir)) {
+      const full = join(imagesDir, name);
+      if (statSync(full).isFile()) {
+        archive.file(full, { name: `images/${name}` });
+      }
+    }
+  }
+
+  void archive.finalize();
+  return archive;
+}
+
+export function backupFilename(): string {
+  const now = new Date();
+  const pad = (n: number) => String(n).padStart(2, '0');
+  const ts = `${now.getFullYear()}${pad(now.getMonth() + 1)}${pad(now.getDate())}_${pad(now.getHours())}${pad(now.getMinutes())}`;
+  return `kochwas-backup_${ts}.zip`;
+}

src/routes/api/admin/backup/+server.ts

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+import type { RequestHandler } from './$types';
+import { createBackupStream, backupFilename } from '$lib/server/backup/export';
+import { Readable } from 'node:stream';
+
+const DB_PATH = process.env.DATABASE_PATH ?? './data/kochwas.db';
+const IMAGE_DIR = process.env.IMAGE_DIR ?? './data/images';
+
+export const GET: RequestHandler = async () => {
+  const archive = createBackupStream({ dbPath: DB_PATH, imagesDir: IMAGE_DIR });
+  const filename = backupFilename();
+  return new Response(Readable.toWeb(archive) as ReadableStream, {
+    status: 200,
+    headers: {
+      'content-type': 'application/zip',
+      'content-disposition': `attachment; filename="${filename}"`
+    }
+  });
+};