- create and edit administrators: password (min 4, max 72 bytes, bcrypt
at admin.password.bcrypt_cost), role picker, radio permission overrides,
superuser switch; no mail on create
- superusers hidden from non-superusers in list, lookups and schemas;
is_superuser projected to superusers so an untouched save keeps it
- save-path rules: superuser-only switch, role and permission ceilings,
no outranking, no own role/flag/overrides/password change, last active
superuser not demoted (advisory lock + count in the write tx)
- password set for another admin bumps tokens_valid_after
- role, superuser, activation and last sign-in list filters
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