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/models/__init__.py

@@ -1,7 +1,7 @@
 from .base import Base
 from .users import User
-from .hosts import Host, ProbeType
-from .monitors import PingResult, DnsResult, PingStatus, DnsStatus
+from .hosts import Host
+from .monitors import Monitor, MonitorResult, MonitorType
 from .metrics import PluginMetric
 from .alerts import AlertRule, AlertState, AlertEvent, AlertOperator, AlertStateEnum
 from .ansible import AnsibleRun, AnsibleRunStatus
@@ -12,8 +12,8 @@ from .dashboard import Dashboard, DashboardWidget, DashboardShareToken
 
 __all__ = [
     "Base", "User",
-    "Host", "ProbeType",
-    "PingResult", "DnsResult", "PingStatus", "DnsStatus",
+    "Host",
+    "Monitor", "MonitorResult", "MonitorType",
     "PluginMetric",
     "AlertRule", "AlertState", "AlertEvent", "AlertOperator", "AlertStateEnum",
     "AnsibleRun", "AnsibleRunStatus",