feat(export): download project photos as a grouped zip

Add POST /api/export {projectIds}: one folder per project (named after it),
photos named <lastname-firstname>.<ext>, with a generated initials SVG for
Members without a Slack photo. Dedupe image fetches, bounded concurrency, fflate
zip. Factor data.ts so routes + export share one fixtures/live gating path.
Frontend: Export-photos button downloads the zip.

backend/package.json

@@ -10,6 +10,7 @@
   },
   "dependencies": {
     "@hono/node-server": "^2.0.6",
+    "fflate": "^0.8.3",
     "hono": "^4.12.27"
   },
   "devDependencies": {