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/models/alerts.py

@@ -55,6 +55,18 @@ class AlertRule(Base):
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, default=lambda: datetime.now(timezone.utc)
     )
+    # What the rule covers (#5464): "resource" watches resource_name itself;
+    # "all_hosts" and "group" make it a covering rule over hosts (every host,
+    # or every host in the inventory group cover_group). A covering rule has
+    # no state and is never evaluated: Steward keeps one ordinary child rule
+    # per covered host (parent_id), so each host's alert keeps its own state,
+    # history and acknowledgement. Fixed when the rule is created.
+    covers: Mapped[str] = mapped_column(String(16), nullable=False, default="resource",
+                                        server_default="resource")
+    cover_group: Mapped[str | None] = mapped_column(String(128), nullable=True)
+    parent_id: Mapped[str | None] = mapped_column(
+        String(36), ForeignKey("alert_rules.id", ondelete="CASCADE"), nullable=True, index=True
+    )
 
 
 class AlertState(Base):