fix(alerts): a metric picked on one part doesn't follow you to another (#5475)

#5475 "Rule form: pick the host, then its part…". CI run 8909 caught it:
moving from the whole host to a mount kept "CPU" in the mount's Metric list,
because a chosen metric always stayed on offer. Only a rule's own page keeps
its resource and metric after they go quiet (keep=True from edit_rule); the
pickers' swaps drop a metric the new part doesn't report.

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

steward/alerts/routes.py

@@ -295,10 +295,12 @@ async def cover_groups_api():
 
 
 async def _target(db, source: str, *, host: str = "", resource: str = "", metric: str = "",
-                  covers: str = "resource") -> dict:
-    """The rule form's target pickers and the chosen metric's preview (#5475)."""
+                  covers: str = "resource", keep: bool = False) -> dict:
+    """The rule form's target pickers and the chosen metric's preview (#5475);
+    `keep` on a rule's own page, so its resource and metric stay on offer."""
     from steward.core.status import sparkline_svg
-    target = await alert_svc.rule_target(db, source, host=host, resource=resource, metric=metric)
+    target = await alert_svc.rule_target(db, source, host=host, resource=resource, metric=metric,
+                                         keep=keep)
     preview = None
     if target["resource"] and target["metric"]:
         preview = await alert_svc.metric_preview(db, source, target["resource"], target["metric"])
@@ -360,7 +362,7 @@ async def edit_rule(rule_id: str):
             return redirect(f"/alerts/rules/{rule.parent_id}/edit")
         ctx = {**await _cover_choices(db, rule.source_module, rule),
                **await _target(db, rule.source_module, resource=rule.resource_name,
-                               metric=rule.metric_name, covers=rule.covers)}
+                               metric=rule.metric_name, covers=rule.covers, keep=True)}
         hosts = sorted((c.resource_name for c in (await db.execute(
             select(AlertRule).where(AlertRule.parent_id == rule.id))).scalars()), key=str.lower)
         history_count = (await db.execute(