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/ping.py

@@ -1,43 +1,17 @@
+"""ICMP / TCP reachability checks — pure probes returning a result dict."""
 from __future__ import annotations
 import asyncio
 import logging
 import time
 
-from sqlalchemy.ext.asyncio import AsyncSession
-
-from steward.core.alerts import record_metric
-from steward.models.hosts import Host, ProbeType
-from steward.models.monitors import PingResult, PingStatus
-
 logger = logging.getLogger(__name__)
 
 TCP_TIMEOUT = 5.0
 DEFAULT_TCP_PORT = 80
 
 
-async def ping_check(host: Host, session: AsyncSession) -> None:
-    """Probe a single host and write ping_result + metrics."""
-    if host.probe_type == ProbeType.icmp:
-        result = await _icmp_ping(host.address)
-    else:
-        result = await _tcp_ping(host.address, host.probe_port or DEFAULT_TCP_PORT)
-
-    status = PingStatus.up if result["up"] else PingStatus.down
-    session.add(PingResult(
-        host_id=host.id,
-        status=status,
-        response_time_ms=result.get("response_time_ms"),
-    ))
-    await session.flush()
-
-    await record_metric(session, "ping", host.name, "up", 1.0 if result["up"] else 0.0)
-    await record_metric(
-        session, "ping", host.name, "response_time_ms",
-        result.get("response_time_ms") or 0.0,
-    )
-
-
-async def _tcp_ping(address: str, port: int) -> dict:
+async def tcp_check(address: str, port: int) -> dict:
+    """TCP connect probe. Returns {is_up, response_ms}."""
     start = time.monotonic()
     try:
         reader, writer = await asyncio.wait_for(
@@ -46,13 +20,17 @@ async def _tcp_ping(address: str, port: int) -> dict:
         )
         writer.close()
         await writer.wait_closed()
-        return {"up": True, "response_time_ms": round((time.monotonic() - start) * 1000, 2)}
+        return {"is_up": True, "response_ms": round((time.monotonic() - start) * 1000, 2)}
     except Exception:
-        return {"up": False, "response_time_ms": None}
+        return {"is_up": False, "response_ms": None}
 
 
-async def _icmp_ping(address: str) -> dict:
-    """Use system ping command (setuid binary; no raw socket privilege needed in Python)."""
+async def icmp_check(address: str) -> dict:
+    """ICMP echo via the system ping binary (setuid; no raw-socket privilege).
+
+    Falls back to a TCP connect on the default port if the ping binary is
+    unavailable or errors, so a host with ICMP filtered still gets a signal.
+    """
     start = time.monotonic()
     try:
         proc = await asyncio.create_subprocess_exec(
@@ -62,8 +40,8 @@ async def _icmp_ping(address: str) -> dict:
         )
         await asyncio.wait_for(proc.wait(), timeout=5.0)
         if proc.returncode == 0:
-            return {"up": True, "response_time_ms": round((time.monotonic() - start) * 1000, 2)}
-        return {"up": False, "response_time_ms": None}
+            return {"is_up": True, "response_ms": round((time.monotonic() - start) * 1000, 2)}
+        return {"is_up": False, "response_ms": None}
     except Exception as exc:
-        logger.warning(f"ICMP ping failed for {address}: {exc}, falling back to TCP")
-        return await _tcp_ping(address, DEFAULT_TCP_PORT)
+        logger.warning("ICMP ping failed for %s: %s, falling back to TCP", address, exc)
+        return await tcp_check(address, DEFAULT_TCP_PORT)