feat(alerts): dispatch after commit; maintenance mutes, not skips; actions via trigger_run (#5063)

Notifications and alert actions now start only once the evaluating
transaction commits, and never for one that rolls back. record_metrics
queues them on the session (_after_commit), and after_commit/after_rollback
listeners on the sync session spawn or drop them. Spawned tasks are held in a
module set so they cannot be garbage-collected before they run.

A maintenance window no longer skips evaluation. Alerts in its scope still
change state, but their notifications and actions are held, and each event
says so ("notification muted: maintenance window <name>"; also on no-data
and resume events). notify_after_maintenance, run every minute with the
silence check as the new core task alert_reconcile, sends one notification
for an alert still firing after its window ends and runs its action. The
event's note records the release.

Alert actions go through runner.trigger_run like every other run path, so
the playbook and the inventory must be ones the source lists (#5034 guards).
The firing event links to the run, or its note says why the action did not
start. The maintenance page and form wording now say what a window does.

Tests:
- integration: a rolled-back evaluation notifies nobody, and dispatch waits
  for the commit; maintenance mutes while the state moves and notifies once
  when the window ends; an action whose playbook is outside its source is
  refused, with the note set;
- the alert-action test uses a listed inventory name and waits, with a
  deadline, for the run the action starts;
- unit: _muted_by.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/alerts/maintenance_form.html

@@ -50,7 +50,7 @@
         <input type="text" name="scope_resource" id="scope-resource"
                placeholder="Exact resource name (e.g. my-server)">
         <p style="font-size:0.78rem;color:var(--text-muted);margin-top:0.25rem;">
-          Leave blank to suppress all resources within the selected source.
+          Leave blank to mute all resources within the selected source.
         </p>
       </div>
 
@@ -64,9 +64,9 @@
   <div class="card" style="margin-top:1rem;">
     <div class="section-title" style="margin-bottom:0.5rem;">How scope works</div>
     <div style="display:grid;gap:0.4rem;font-size:0.82rem;color:var(--text-muted);">
-      <div><strong style="color:var(--text);">All alerts</strong> — every alert rule is suppressed</div>
-      <div><strong style="color:var(--text);">By source</strong> — e.g. source=ping suppresses all ping alerts</div>
-      <div><strong style="color:var(--text);">By source + resource</strong> — e.g. ping/my-server suppresses only that host's ping alerts</div>
+      <div><strong style="color:var(--text);">All alerts</strong> — every alert rule's notifications are muted</div>
+      <div><strong style="color:var(--text);">By source</strong> — e.g. source=icmp mutes all ping alerts</div>
+      <div><strong style="color:var(--text);">By source + resource</strong> — e.g. icmp/my-server mutes only that host's ping alerts</div>
     </div>
   </div>
 </div>