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>

steward/hosts/service.py

@@ -113,6 +113,17 @@ async def host_status(db, host_ids: list[str]) -> dict[str, dict]:
     return out
 
 
+def worst_uptime_first(hosts, uptime: dict) -> list:
+    """Hosts by their 24h uptime, worst first, then 7d and 30d to break ties;
+    hosts with no uptime yet (no monitors) last, by name."""
+    def key(h):
+        u = uptime.get(h.id) or {}
+        windows = [u.get(w) for w in ("24h", "7d", "30d")]
+        known = [w for w in windows if w is not None]
+        return (not known, *[101.0 if w is None else w for w in windows], h.name.lower())
+    return sorted(hosts, key=key)
+
+
 def host_rank(status: dict | None, agent: dict | None) -> int:
     """Where a host sorts in the Hosts list: a down monitor first, then an
     agent gone quiet, then monitors not yet checked, then the healthy; a host
@@ -137,16 +148,23 @@ async def host_monitors(db, host_id: str) -> list[dict]:
     recent = await _last_n_results(db, ids)
     uptime = await _uptime_by_monitor(db, ids)
     data = []
+    from steward.monitors.service import down_reason, down_since
     for m in monitors:
         mrows = recent.get(m.id, [])
         latest = mrows[-1] if mrows else None
+        state = ("paused" if not m.enabled else "pending" if latest is None
+                 else "up" if latest.is_up else "down")
         data.append({
-            "monitor": m, "latest": latest,
+            "monitor": m, "latest": latest, "state": state,
             "heartbeat": [{"state": "up" if r.is_up else "down",
                            "title": _heartbeat_title(m.type, r)} for r in mrows],
             "uptime_pct": uptime.get(m.id, {}).get("24h"),
+            "down_since": await down_since(db, m.id) if state == "down" else None,
+            "down_reason": down_reason(latest) if state == "down" else None,
         })
-    return data
+    # Problems first: down, then not yet checked, then up, paused last.
+    order = {"down": 0, "pending": 1, "up": 2, "paused": 3}
+    return sorted(data, key=lambda d: (order[d["state"]], d["monitor"].name.lower()))
 
 
 async def agent_overview_by_host(