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/plugins/traefik/test_traefik_page.py

@@ -5,7 +5,7 @@ from types import SimpleNamespace as NS
 
 import pytest
 
-from plugins.traefik.routes import router_state, split_routers
+from plugins.traefik.routes import gone_routers, router_state, split_routers
 from steward.core.settings import to_thresholds_cfg
 from tests._templates import render_plugin_template
 from tests.test_style_tokens import hex_colours
@@ -63,3 +63,28 @@ async def test_the_page_leads_with_problems_and_folds_the_healthy(app):
     assert page.index(">docs</td>") < page.index(">web</td>")
     assert 'class="tone-crit">2 days</td>' in page and 'class="tone-ok">60 days</td>' in page
     assert "font-weight:600" not in page and not hex_colours(page), hex_colours(page)
+
+
+def test_a_router_traefik_stopped_reporting_is_set_apart_not_failing():
+    live = _router("api", _latest())
+    removed = _router("old", _latest(e5=9.0))
+    removed["latest"].scraped_at = live["latest"].scraped_at - timedelta(minutes=10)
+    recent = _router("blip", _latest())
+    recent["latest"].scraped_at = live["latest"].scraped_at - timedelta(seconds=60)
+    current, gone = gone_routers([live, removed, recent], 30)     # 3 scrapes = 90 s
+    assert [r["name"] for r in current] == ["api", "blip"]
+    assert [r["name"] for r in gone] == ["old"]
+    assert gone_routers([], 30) == ([], [])
+
+
+async def test_gone_routers_are_listed_by_when_they_were_last_seen(app):
+    removed = _router("old", _latest(e5=9.0))
+    removed["latest"].scraped_at = datetime.now(timezone.utc) - timedelta(hours=2)
+    async with app.test_request_context("/plugins/traefik/rows"):
+        page = await render_plugin_template(app, "traefik", "traefik/rows.html", problems=[], healthy=[],
+                                            gone=[removed], global_stat=None, certs=[], range_key="24h",
+                                            error_4xx=1.0, error_5xx=0.1)
+    page = " ".join(page.split())
+    assert "1 router no longer reported</summary>" in page
+    assert "old</span> <span class=\"problem-detail\">last seen <time" in page and ">2h ago</time>" in page
+    assert "Failing" not in page