* fix(auth): validate IdP authority before SAML account linking
Adds verification that SAML IdP is authoritative for the email domain
before allowing account conversion
* fix(auth): deny by default on missing SAML tenant + optimize membership query
- Block account conversion when tenant is missing (deny by default)
- Replace JOIN + ILIKE with two indexed lookups for O(1) performance
* refactor(auth): apply data minimization to security logs