feat(ui): host page leads with its alerts; old readings say how old (#5210)

#5210 "Problems first, with since-time, across every list" (milestone 477),
second batch.

- Host page: an "Alerts on this host" strip first (rules on its agent
  series, its containers or its monitors: alert_svc.watches_host); its
  monitors problems first with "Down 12m · reason"; Docker above Ansible.
- Uptime page and widget: worst 24h uptime first, then 7d and 30d; hosts
  with no uptime yet last.
- Agent panel and fleet widget: last seen as "3m ago", not a clock time.
- SNMP: a device is "Reachable" only while it answered within the last 3
  polls; older readings say "No answer" with their age, and every device
  says when it was last read (was: reachable whenever any reading existed).
- Traefik: a router missing from 3 scrapes in a row is listed apart as no
  longer reported, by when it was last seen, instead of keeping its last
  state (a removed router stayed Failing).

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

tests/core/test_problems_first.py

@@ -29,3 +29,27 @@ def test_schedules_sort_failing_first_and_paused_last():
             _sched("busy", last_error="the last run is still going", status="success")]
     assert [r["s"].name for r in sorted(rows, key=_schedule_order)] == [
         "backup", "broken", "cut", "deploy", "busy", "nightly", "paused"]
+
+
+def test_a_host_page_shows_only_the_alerts_on_that_host():
+    from steward.alerts.service import watches_host
+
+    def rule(source, resource):
+        return NS(source_module=source, resource_name=resource)
+    mine = {("http", "nas web")}
+    assert watches_host(rule("host_agent", "nas"), "nas", mine)
+    assert watches_host(rule("host_agent", "nas:/"), "nas", mine)
+    assert not watches_host(rule("host_agent", "nas2"), "nas", mine)       # a name that merely starts alike
+    assert watches_host(rule("docker", "nas/plex"), "nas", mine)
+    assert not watches_host(rule("docker", "nas2/plex"), "nas", mine)
+    assert watches_host(rule("http", "nas web"), "nas", mine)
+    assert not watches_host(rule("http", "other web"), "nas", mine)
+
+
+def test_uptime_lists_the_worst_first_and_the_unmeasured_last():
+    from steward.hosts.service import worst_uptime_first
+    hosts = [NS(id=i, name=n) for i, n in [("a", "alpha"), ("b", "bravo"), ("c", "charlie"), ("d", "delta")]]
+    uptime = {"a": {"24h": 100.0, "7d": 99.0, "30d": 99.5},
+              "b": {"24h": 92.0, "7d": 98.0, "30d": 99.0},
+              "c": {"24h": 100.0, "7d": 95.0, "30d": 97.0}}
+    assert [h.name for h in worst_uptime_first(hosts, uptime)] == ["bravo", "charlie", "alpha", "delta"]