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/core/cleanup.py

@@ -5,7 +5,7 @@ from typing import TYPE_CHECKING
 
 from sqlalchemy import delete
 
-from steward.models.monitors import DnsResult, PingResult
+from steward.models.monitors import MonitorResult
 from steward.models.metrics import PluginMetric
 from steward.models.ansible import AnsibleRun
 
@@ -23,8 +23,7 @@ async def run_cleanup(app: "Quart") -> None:
     async with app.db_sessionmaker() as session:
         async with session.begin():
             for model, ts_col in [
-                (PingResult, PingResult.probed_at),
-                (DnsResult, DnsResult.resolved_at),
+                (MonitorResult, MonitorResult.checked_at),
                 (PluginMetric, PluginMetric.recorded_at),
                 (AnsibleRun, AnsibleRun.started_at),
             ]: