Ack System Broadcast

Record "Li e compreendi" for the current user. Any authenticated user.

Idempotent: a double-click, a retried request and a second tab all leave exactly one receipt (enforced by UX_Z_SystemBroadcastAcks_pair, not by a read-then-write that would race).

Always 200. recorded: false means the receipt did not stick — the acks table is not migrated, or the caller has no resolvable user identity — and the announcement will simply be offered again next login. Seeing a message twice is the correct failure; never showing it is not, and neither is blocking the dialog's close button on a database write.

NOTE this is deliberately NOT system_admin-gated and NOT in platform_paths.py: it is an ordinary user acting for themselves, and its audit row belongs to their tenant.