feat(alerts): pick the host, then its part, and see what the metric reads (#5475)

#5475 "Rule form: pick the host, then its part; metrics that part reports;
a link to groups; a preview of the metric", from the operator's review of
the New rule page: hosts and their parts shared one long picker.

- Host agent rules pick a Host, then a Part: "The whole host", then its
  mounts, CPU cores, disks, interfaces and sensors under a heading each,
  cores in number order. A part that reports nothing a rule can watch
  (an interface's traffic) isn't offered.
- The Metric list holds what the picked resource reports, in catalog order.
- Under it, a preview: the reading now, and the last 24 h low to high with
  an hourly sparkline drawn from zero (UI ruling 8), read from one series.
- The group picker links to Ansible > Inventory to make a group, in a new
  tab, and re-reads the groups when the tab comes back into view.
- Picking a host or part swaps the pickers alone (/alerts/api/rule_target);
  picking a metric swaps the preview (/alerts/api/metric_preview).
- A rule's own resource and metric stay on offer after they go quiet.

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

tests/core/test_alerts_page.py

@@ -119,7 +119,8 @@ async def test_deleting_a_rule_is_on_its_own_page_and_says_what_goes(app):
         listing = await render_template("alerts/list.html", active=[], rules=[rule], operators=[])
         page = await render_template(
             "alerts/rules_form.html", rule=rule, operators=[], source_modules=["host_agent"],
-            initial_source="host_agent", resources=["web-1"], metrics=["cpu_pct"], ansible_sources=[],
+            initial_source="host_agent", resources=["web-1"], resource="web-1", metrics=["cpu_pct"],
+            metric="cpu_pct", ansible_sources=[],
             catalog={}, history_count=12, fired=None, notify=[])
     assert "/delete" not in listing
     assert 'action="/alerts/rules/r1/delete"' in page
@@ -138,7 +139,8 @@ async def test_the_rule_page_is_named_for_the_rule(app):
         session["user_role"] = "admin"
         page = await render_template(
             "alerts/rules_form.html", rule=rule, operators=[(">", ">")], source_modules=["host_agent"],
-            initial_source="host_agent", resources=["VDNT-Docker02"], metrics=["seconds_since_seen"],
+            initial_source="host_agent", resources=["VDNT-Docker02"], resource="VDNT-Docker02",
+            metrics=["seconds_since_seen"], metric="seconds_since_seen",
             ansible_sources=[], catalog={}, history_count=10,
             fired=datetime.now(timezone.utc) - timedelta(hours=6), notify=["email to me@example.com"])
     page = " ".join(page.split())
@@ -265,8 +267,8 @@ async def test_the_rule_form_speaks_in_names_and_units(app):
         session["user_role"] = "operator"
         page = await render_template(
             "alerts/rules_form.html", rule=rule, operators=_OPERATORS,
-            source_modules=["http", "host_agent"], initial_source="http", resources=["nas"],
-            metrics=["is_up", "response_ms"], ansible_sources=[],
+            source_modules=["http", "host_agent"], initial_source="http", resources=["nas"], resource="nas",
+            metrics=["is_up", "response_ms"], metric="is_up", ansible_sources=[],
             catalog={"http": ["is_up", "response_ms"], "snmp": []})
     assert ">HTTP monitors</option>" in page and ">Host agent</option>" in page
     assert ">Up or down</option>" in page and 'data-unit="ms"' in page
@@ -294,7 +296,8 @@ async def test_a_rule_covering_hosts_lists_its_hosts_under_it(app):
         page = await render_template(
             "alerts/rules_form.html", rule=parent, operators=[], source_modules=["host_agent"],
             initial_source="host_agent", resources=[], metrics=["seconds_since_seen"],
-            ansible_sources=[], catalog={}, history_count=0, fired=None, notify=[],
+            metric="seconds_since_seen", ansible_sources=[], catalog={}, history_count=0,
+            fired=None, notify=[],
             covered=["node-a", "node-b"], host_source=True, groups=["swarm"],
             current_covers="group", current_group="swarm", covers_locked=True)
     listing, page = " ".join(listing.split()), " ".join(page.split())