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/monitors/runner.py

@@ -0,0 +1,56 @@
+"""Unified check dispatcher: Monitor -> normalised MonitorResult fields.
+
+One entry point, `run_monitor`, dispatches on Monitor.type to the per-type
+probes (ping/dns/http) and returns a dict with every MonitorResult field so the
+scheduler can persist a row uniformly regardless of type.
+"""
+from __future__ import annotations
+import logging
+
+from steward.models.monitors import Monitor, MonitorType
+from .ping import tcp_check, icmp_check, DEFAULT_TCP_PORT
+from .dns import dns_check
+from .http import http_check
+
+logger = logging.getLogger(__name__)
+
+# Every result carries these keys; type-specific probes fill the relevant ones.
+_BLANK = {
+    "is_up": False,
+    "response_ms": None,
+    "status_code": None,
+    "resolved_ip": None,
+    "content_matched": None,
+    "tls_expires_at": None,
+    "error_msg": None,
+}
+
+
+async def run_monitor(monitor: Monitor) -> dict:
+    """Run one monitor's check and return a full MonitorResult field dict."""
+    cfg = monitor.config
+    result = dict(_BLANK)
+    try:
+        if monitor.type == MonitorType.icmp.value:
+            result.update(await icmp_check(monitor.target))
+        elif monitor.type == MonitorType.tcp.value:
+            result.update(await tcp_check(monitor.target, int(cfg.get("port") or DEFAULT_TCP_PORT)))
+        elif monitor.type == MonitorType.dns.value:
+            result.update(await dns_check(monitor.target, cfg.get("expected_ip") or None))
+        elif monitor.type == MonitorType.http.value:
+            result.update(await http_check(
+                url=monitor.target,
+                method=cfg.get("method", "GET"),
+                expected_status=int(cfg.get("expected_status", 200) or 200),
+                content_match=cfg.get("content_match", "") or "",
+                headers=cfg.get("headers") or {},
+                timeout_seconds=int(cfg.get("timeout_seconds", 10) or 10),
+                follow_redirects=bool(cfg.get("follow_redirects", True)),
+                verify_ssl=bool(cfg.get("verify_ssl", True)),
+            ))
+        else:
+            result["error_msg"] = f"Unknown monitor type {monitor.type!r}"
+    except Exception as exc:  # a probe should never take the scheduler down
+        logger.exception("Monitor %r (%s) check raised", monitor.name, monitor.type)
+        result["error_msg"] = str(exc)[:512]
+    return result