* Fail closed on delegated fetch backends to close SSRF gap
* fix(community): resolve delegated-fetch credentials from one config snapshot
Address review feedback on the fail-closed SSRF guard:
- firecrawl/fastcrw client construction no longer re-reads get_app_config(),
so the API key and endpoint come from the same revision the endpoint screen
validated (prevents cross-revision credential misrouting).
- fix the browserless test double to accept the new two-argument
_get_browserless_client signature.
- document the check-only (DNS rebinding) caveat on
validate_delegated_backend_url.
- add a CHANGELOG upgrade note for the default self-hosted backend break.
* fix(doctor): consolidate delegated fetch backend checks and deployment guidance
---------
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>