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/alerts/service.py

@@ -215,6 +215,18 @@ async def active_rows(db, now: datetime) -> list[dict]:
     return rows
 
 
+def watches_host(rule: AlertRule, host_name: str, monitor_keys: set) -> bool:
+    """Whether a rule watches something on this host: its agent series
+    ("<host>" or "<host>:<part>"), one of its containers ("<host>/<name>"),
+    or one of its monitors ((type, name) in monitor_keys)."""
+    source, resource = rule.source_module, rule.resource_name
+    if source == "host_agent":
+        return resource == host_name or resource.startswith(host_name + ":")
+    if source == "docker":
+        return resource.startswith(host_name + "/")
+    return (source, resource) in monitor_keys
+
+
 async def active_count(db) -> int:
     """How many alerts need attention: the sidebar's number."""
     from sqlalchemy import func