PUBLIC_AE_BOOTSTRAP_KEY replaces the hardcoded 'IDF68Em5X4HTZlswRNgepQ' in:
- src/routes/+layout.ts (site-domain bootstrap request)
- src/routes/testing/+page.svelte (trace agent key)
Added to .env.staging, .env.prod, .env.local (gitignored), and updated
.env.staging.default / .env.prod.default with XXXX placeholders.
Key can now be rotated independently from the main API secret key.