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/uptime.html

@@ -7,13 +7,10 @@
 </div>
 
 {# ── Summary pills ──────────────────────────────────────────────────────────── #}
-{% set ping_hosts = hosts | selectattr("ping_enabled") | list %}
-{% set total = ping_hosts | length %}
+{% set total = hosts | length %}
 {% if total %}
-{% set full_30d = ping_hosts | selectattr("id", "in", uptime.keys())
-                             | selectattr("id", "defined") | list %}
 {% set healthy = namespace(count=0) %}
-{% for h in ping_hosts %}
+{% for h in hosts %}
   {% set pct = uptime.get(h.id, {}).get("30d") %}
   {% if pct is not none and pct >= 99 %}{% set healthy.count = healthy.count + 1 %}{% endif %}
 {% endfor %}
@@ -50,15 +47,13 @@
       {% for host in hosts %}
       {% set ut = uptime.get(host.id, {}) %}
       <tr>
-        <td style="font-weight:500;">{{ host.name }}</td>
+        <td style="font-weight:500;"><a href="/hosts/{{ host.id }}" style="color:inherit;">{{ host.name }}</a></td>
         <td style="color:var(--text-muted);font-size:0.85rem;">{{ host.address }}</td>
 
         {% for window in ["24h", "7d", "30d"] %}
         {% set pct = ut.get(window) %}
         <td style="text-align:center;font-size:0.88rem;font-family:ui-monospace,monospace;">
-          {% if not host.ping_enabled %}
-            <span style="color:var(--text-dim);">—</span>
-          {% elif pct is none %}
+          {% if pct is none %}
             <span style="color:var(--text-muted);" title="No data yet">—</span>
           {% elif pct >= 99.9 %}
             <span style="color:var(--green);">{{ "%.2f"|format(pct) }}%</span>
@@ -75,9 +70,7 @@
         {# 30-day visual bar #}
         {% set pct30 = ut.get("30d") %}
         <td>
-          {% if not host.ping_enabled %}
-            <span style="color:var(--text-dim);font-size:0.8rem;">ping off</span>
-          {% elif pct30 is none %}
+          {% if pct30 is none %}
             <span style="color:var(--text-muted);font-size:0.8rem;">no data</span>
           {% else %}
           <div style="display:flex;align-items:center;gap:0.5rem;">
@@ -99,6 +92,6 @@
 {% endif %}
 
 <p style="color:var(--text-dim);font-size:0.78rem;margin-top:1rem;">
-  Uptime is computed from ping probe results only. Hosts with ping disabled show —.
+  Uptime aggregates every monitor attached to a host. Hosts with no monitors show —.
 </p>
 {% endblock %}