Broadcast audits a half-failed send as a success

[Taure]

asobi_admin_notifications_controller:broadcast/1 calls
asobi_admin_ui_audit:log_action(~"broadcast", #{recipient_count => length(SentTo)}, {ok, SentTo}).

The {ok, SentTo} is constructed by the handler, not returned by the
operation. asobi_admin_notifications:do_broadcast/4 drops every failed
insert on the floor and returns only the survivors, so a broadcast to ten
players that reached three is logged as outcome => ok with
recipient_count => 3. Nothing anywhere records that seven failed.

What core now provides

asobi core ships the durable audit and the widened contract (plan item 2a.8):

• asobi_notify:send_many/4 returns {ok, Succeeded, Failed}.
• asobi_ops_notifications:broadcast/5 takes an actor, fans out, and writes
  an ops_audit_entries row through asobi_ops_audit:mutation/4. It cannot
  misreport, because the row is built from the operation's own return value.
• The row carries actor_id, actor_display, actor_source and
  actor_attested, plus outcome (ok / partial / error) and the two
  counts.

Work here

1. Delete asobi_admin_notifications:do_broadcast/4 - it is a copy of
   asobi_notify:send_many/4 and now a stale one.
2. Have the UI controller and asobi_admin_notifications:broadcast/1 call
   asobi_ops_notifications:broadcast/5, passing the session's actor.
3. Drop the hand-built {ok, SentTo} audit call at the broadcast site.
4. Decide what the UI shows on a partial send. The current redirect carries
   only ?sent=N, which reads as a success.

asobi_admin has no actor record yet; it still pins one hardcoded principal.
Constructing an #{id, display, source, caps, attested} for the session is
part of this, or a prerequisite issue.