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/form.html

@@ -15,47 +15,16 @@
                 <label>Address (hostname or IP)</label>
                 <input type="text" name="address" value="{{ host.address if host else '' }}" required>
             </div>
-            <div class="form-group">
-                <label>Probe Type</label>
-                <select name="probe_type">
-                    {% for pt in probe_types %}
-                    <option value="{{ pt.value }}" {% if host and host.probe_type == pt %}selected{% endif %}>
-                        {{ pt.value | upper }}
-                    </option>
-                    {% endfor %}
-                </select>
-            </div>
-            <div class="form-group">
-                <label>TCP Port (used for TCP probe; ignored for ICMP)</label>
-                <input type="number" name="probe_port" value="{{ host.probe_port if host else 80 }}" min="1" max="65535">
-            </div>
-            <div class="form-group">
-                <label style="display:flex;align-items:center;gap:0.5rem;cursor:pointer;">
-                    <input type="checkbox" name="ping_enabled" {% if not host or host.ping_enabled %}checked{% endif %}>
-                    Enable ping monitor
-                </label>
-            </div>
-            <div class="form-group">
-                <label style="display:flex;align-items:center;gap:0.5rem;cursor:pointer;">
-                    <input type="checkbox" name="dns_enabled" {% if host and host.dns_enabled %}checked{% endif %}>
-                    Enable DNS monitor
-                </label>
-            </div>
-            <div class="form-group">
-                <label>Expected IP (optional — leave blank to accept any)</label>
-                <input type="text" name="dns_expected_ip" value="{{ host.dns_expected_ip if host and host.dns_expected_ip else '' }}">
-            </div>
             <div style="display:flex;gap:1rem;margin-top:1.5rem;">
                 <button type="submit" class="btn">{% if host %}Save{% else %}Add Host{% endif %}</button>
-                <a href="/hosts/" class="btn btn-ghost">Cancel</a>
+                <a href="{% if host %}/hosts/{{ host.id }}{% else %}/hosts/{% endif %}" class="btn btn-ghost">Cancel</a>
             </div>
         </form>
     </div>
-    {% if host %}
     <p style="color:var(--text-muted);font-size:0.82rem;margin-top:1rem;text-align:center;">
-        Agent, Ansible target, and playbook runs live on the
-        <a href="/hosts/{{ host.id }}">host page</a>.
+        {% if host %}Monitors, agent, Ansible target, and playbook runs live on the
+        <a href="/hosts/{{ host.id }}">host page</a>.{% else %}
+        After adding, attach ping / DNS / HTTP monitors from the host page.{% endif %}
     </p>
-    {% endif %}
 </div>
 {% endblock %}