CVE-2026-71303: Lemur: Incomplete fix for GHSA-v2wp-frmc-5q3v -- ACME authority update endpoint allows non-admin to replace `acme_url` with internal IP, bypassing allowlist

The fix for GHSA-v2wp-frmc-5q3v added _validate_acme_url() to reject acme_url values not in ACME_DIRECTORY_HOST_ALLOWLIST, but the validation is only called at authority creation time (POST). The authority update endpoint (PUT /api/1/authorities/<id>) accepts and stores arbitrary options – including a modified acme_url – without invoking the allowlist check. Any user with an authority role (granted by an admin to allow issuing certificates via that authority) can therefore overwrite the stored acme_url with an internal IP or IMDS endpoint. The next certificate issuance via that authority causes Lemur’s backend to fetch the attacker-controlled URL, achieving SSRF.