feat(alerts): one rule can cover every host, or every host in a group (#5464)

#5464 "Group rules: one rule covers every host, or every host in a group",
step 2 of milestone 499 "Alerts you can trust". Each rule named one host, so
the live instance carried six rules for three hosts, and a new host had no
agent-offline or disk alert until someone copied them.

- alert_rules.covers / cover_group / parent_id (migration 0041). A rule on
  host_agent may cover "all_hosts" or a "group" (an inventory group: the
  Hosts ruling, #5294). A covering rule has no state; Steward keeps one
  ordinary child rule per covered host, named "<rule>: <host>", so each
  host's alert keeps its own state, history and acknowledgement.
- alerts.service.reconcile_children: a covered host that has reported in
  the last 7 days gets a child; a host no longer covered loses its child;
  children take the parent's condition, checks, notify setting, action and
  on/off. Run on save and each minute (alert_reconcile).
- A child is changed through its parent: edit, toggle and delete of a child
  are refused (409 / PartOfCoveringRule); its edit page opens the parent.
- Rule form: a Covers choice for host sources, with the group picker; fixed
  once the rule exists. Rules list: a covering rule reads "every host in
  swarm · 3 hosts" with its hosts' rules nested under it, each opening its
  history. The history picker and the rule count leave covering rules out.
- MCP: create_alert_rule takes covers/group; update_alert_rule takes group
  and passes changes down; list_alert_rules shows covers, hosts and part_of.
- docs/core/alerting.md: "Rules that cover hosts".

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

steward/app.py

@@ -408,8 +408,9 @@ def _register_core_tasks(app: Quart) -> None:
         await _cleanup(app)
 
     async def run_alert_reconcile():
+        from .alerts.service import reconcile_covering_rules
         from .core.alerts import check_silence, notify_after_maintenance
-        for step in (check_silence, notify_after_maintenance):
+        for step in (reconcile_covering_rules, check_silence, notify_after_maintenance):
             try:
                 await step(app)
             except Exception:
@@ -439,7 +440,8 @@ def _register_core_tasks(app: Quart) -> None:
             interval_seconds=60,
             run_on_startup=True,
         ),
-        # Rules whose series has gone quiet get a visible no_data mark; alerts
+        # Covering rules gain a child for each newly covered host (#5464);
+        # rules whose series has gone quiet get a visible no_data mark; alerts
         # a maintenance window muted notify once it ends.
         ScheduledTask(
             name="alert_reconcile",