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/service.py

@@ -391,13 +391,14 @@ def alertable(source_module: str, reported: set[str]) -> list[str]:
 
 
 async def rule_target(db, source_module: str, *, host: str = "", resource: str = "",
-                      metric: str = "") -> dict:
+                      metric: str = "", keep: bool = False) -> dict:
     """What the rule form's target pickers offer (#5475), from the latest-value
     table: the resources that report something a rule can watch, and for the
     chosen one the metrics it reports. For a host source the resources split
     into a Host picker and a Part picker ("the whole host", then the host's
-    mounts, cores and so on). A resource or metric already chosen stays on
-    offer even when it has gone quiet, so editing a rule never drops them.
+    mounts, cores and so on). With `keep` (a rule's own page), its resource
+    and metric stay on offer even after they go quiet, so opening a rule
+    never drops them; a metric carried over from another part is dropped.
 
     Returns host_source, hosts, host, parts [(heading, [(value, label)])],
     resources, resource, metrics, metric."""
@@ -434,7 +435,7 @@ async def rule_target(db, source_module: str, *, host: str = "", resource: str =
     else:
         out["resources"] = sorted(pickable, key=str.lower)
     metrics = list(pickable.get(out["resource"]) or catalog)
-    if metric and metric not in metrics and (metric in catalog or not catalog):
+    if keep and metric and metric not in metrics and (metric in catalog or not catalog):
         metrics.append(metric)
     out.update(metrics=metrics, metric=metric if metric in metrics else "")
     return out