feat(monitors): unify ping/dns/http into one Monitor entity + custom targets

Collapse the three former check types into a single core `Monitor` entity
with one management surface (/monitors), one result table (monitor_results),
and a single scheduled task. Every type can now watch a free-standing custom
destination (optional host_id) — not just a registered Host.

- models: Monitor + MonitorResult replace PingResult/DnsResult; Host loses its
  ping/dns facet columns (now Monitor rows linked by host_id).
- checks: monitors/{ping,dns,http}.py pure probes + runner.run_monitor
  dispatcher; one monitor_check scheduler with a per-monitor due-filter.
- status: single monitor_status_source replaces the three sources.
- UI: /monitors blueprint (type-aware add/edit/list/widget); host hub shows a
  host's linked monitors + "add monitor for this host"; nav + widget registry
  + alert metric catalog rewired. http plugin folded into core and removed.
- migration 0022 merges the http branch, data-migrates host facets +
  http_monitors + all three result histories, drops the old tables/columns.

Resolves the per-host ping/dns auto-attach issue (#275): monitors are now
explicit, never auto-added to every host.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

steward/templates/hosts/overview_widget.html

@@ -4,18 +4,17 @@
 {% if hosts %}
 <div class="host-blocks">
   {% for host in hosts %}
-  {% set ping = latest_pings.get(host.id) %}
-  {% set dns = latest_dns.get(host.id) %}
+  {% set hs = host_status.get(host.id) %}
   {% set ut = uptime.get(host.id, {}) %}
   {% set a = agent.get(host.name, {}) %}
-  {% set down = host.ping_enabled and ping and ping.status.value != 'up' %}
+  {% set down = hs and hs.state == 'down' %}
   {% set spark = cpu_sparks.get(host.name) %}
   <div style="padding:0.5rem 0;border-bottom:1px solid var(--border);">
     {# Line 1 — status dot + full host name (own line) + cpu trend #}
     <div style="display:flex;align-items:center;gap:0.5rem;">
-      <span class="dot {% if down %}dot-down{% elif host.ping_enabled and ping %}dot-up{% else %}dot-dim{% endif %}"
+      <span class="dot {% if down %}dot-down{% elif hs and hs.state == 'up' %}dot-up{% else %}dot-dim{% endif %}"
             style="flex-shrink:0;"
-            title="{% if not host.ping_enabled %}ping off{% elif ping %}{{ ping.status.value }}{% else %}no data{% endif %}"></span>
+            title="{% if not hs %}no monitors{% else %}{{ hs.state }}{% endif %}"></span>
       {% if session.user_id %}
       <a href="/hosts/{{ host.id }}" style="font-weight:600;font-size:0.85rem;overflow-wrap:anywhere;">{{ host.name }}</a>
       {% else %}
@@ -25,8 +24,8 @@
     {# Line 2 — monitors + agent metrics; wraps instead of truncating #}
     <div style="display:flex;flex-wrap:wrap;gap:0.25rem 0.9rem;margin-top:0.25rem;padding-left:1.1rem;
                 font-variant-numeric:tabular-nums;color:var(--text-muted);font-size:0.75rem;">
-      {% if host.ping_enabled and ping and ping.response_time_ms is not none %}
-      <span title="Ping latency" style="{{ threshold_style(ping.response_time_ms, 'latency') }}"><span style="color:var(--text-dim);">ping</span> {{ "%.0f"|format(ping.response_time_ms) }}ms</span>
+      {% if hs and hs.latency_ms is not none %}
+      <span title="Latency" style="{{ threshold_style(hs.latency_ms, 'latency') }}"><span style="color:var(--text-dim);">ping</span> {{ "%.0f"|format(hs.latency_ms) }}ms</span>
       {% endif %}
       {% if ut.get('24h') is not none %}
       <span title="Uptime 24h" style="{{ threshold_style(ut['24h'], 'uptime') }}"><span style="color:var(--text-dim);">24h</span> {{ "%.1f"|format(ut['24h']) }}%</span>