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/integration/test_alert_rule_form.py

@@ -0,0 +1,67 @@
+"""Integration: the rule form's host and part pickers and the metric preview
+(#5475), through their routes against Postgres. A host's parts come grouped
+under it, and a picked metric shows what it reads now and over the last day."""
+from __future__ import annotations
+
+import asyncio
+import os
+import time
+import uuid
+from datetime import datetime, timedelta, timezone
+
+import pytest
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.fixture
+def app():
+    if not os.environ.get("STEWARD_DATABASE_URL"):
+        pytest.skip("needs Postgres")
+    from steward.app import create_app
+    return create_app(testing=False)
+
+
+def test_the_pickers_and_the_preview_read_what_the_host_reports(app):
+    from steward.core.alerts import record_metrics
+    from steward.models.metrics import PluginMetric
+    from steward.models.users import User, UserRole
+
+    async def _go():
+        tag = uuid.uuid4().hex[:8]
+        host = f"rf-{tag}"
+        now = datetime.now(timezone.utc)
+        async with app.db_sessionmaker() as s:
+            async with s.begin():
+                user = User(username=f"rf-{tag}", email=f"rf-{tag}@test", password_hash="x",
+                            role=UserRole.operator, is_active=True)
+                s.add(user)
+                # Disk use climbing over the last few hours, then cpu and a mount.
+                await record_metrics(s, [
+                    PluginMetric(source_module="host_agent", resource_name=f"{host}:/data",
+                                 metric_name="disk_used_pct", value=v, recorded_at=now - timedelta(hours=h))
+                    for h, v in ((5, 61.0), (3, 70.0), (1, 82.0))])
+                await record_metrics(s, [
+                    PluginMetric(source_module="host_agent", resource_name=host,
+                                 metric_name="cpu_pct", value=12.0, recorded_at=now)])
+            uid = user.id
+        client = app.test_client()
+        async with client.session_transaction() as sess:
+            sess.update(user_id=uid, user_role="operator", username=f"rf-{tag}",
+                        _csrf="test-token", login_at=time.time())
+
+        resp = await client.get("/alerts/api/rule_target", query_string={
+            "source_module": "host_agent", "pick_host": host})
+        page = " ".join((await resp.get_data(as_text=True)).split())
+        assert resp.status_code == 200
+        assert f'<option value="{host}" selected>The whole host</option>' in page
+        assert f'<optgroup label="Mounts"> <option value="{host}:/data" >/data</option>' in page
+
+        resp = await client.get("/alerts/api/metric_preview", query_string={
+            "source_module": "host_agent", "pick_host": host, "resource_name": f"{host}:/data",
+            "metric_name": "disk_used_pct"})
+        page = " ".join((await resp.get_data(as_text=True)).split())
+        assert "Now <strong>82%</strong>" in page
+        assert "last 24 h</span> 61% – 82%" in page and "<polyline" in page
+
+    asyncio.run(_go())