Legal holds, step 1: the hold itself

inbuxa:LegalHold get/set places a hold on accounts, groups, domains,
tenants or the whole server, with an optional date range. A hold's range
and scope can only widen, a released hold is read-only, and none is ever
deleted. Placing, changing and releasing each need a reason and are
audited (LH-1, LH-3, LH-10, AU-12).

Permissions 669-672 (see, place, widen or release, export held data)
go to server administrators only; the tenant ceiling always strips them,
as it does Impersonate (LH-13). Schema: Compliance > Legal Holds.

What a hold keeps comes next, through the undelete hooks.

Also moves the lock expiry helpers below the lock module's imports.

tests/src/system/mod.rs

@@ -12,6 +12,7 @@ pub mod ai;
 pub mod ai_calibration;
 pub mod ai_explain;
 pub mod account_lock; // inbuxa: account lock with delegation
+pub mod legal_hold; // inbuxa: legal hold
 pub mod audit; // inbuxa: the audit log
 pub mod authorization;
 pub mod auto_reload; // inbuxa: registry writes apply at once