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/hosts.py

@@ -1,29 +1,20 @@
 from __future__ import annotations
-import enum
 import uuid
 from datetime import datetime, timezone
-from sqlalchemy import Boolean, DateTime, Enum, Integer, String
+from sqlalchemy import DateTime, String
 from sqlalchemy.orm import Mapped, mapped_column
 from .base import Base
 
 
-class ProbeType(str, enum.Enum):
-    tcp = "tcp"
-    icmp = "icmp"
-
-
 class Host(Base):
+    """A registered host. Reachability/DNS/HTTP checks are now separate Monitor
+    rows that link back via Monitor.host_id — a Host is just identity + address.
+    """
     __tablename__ = "hosts"
 
     id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
     name: Mapped[str] = mapped_column(String(128), nullable=False)
     address: Mapped[str] = mapped_column(String(255), nullable=False)
-    probe_type: Mapped[ProbeType] = mapped_column(Enum(ProbeType), nullable=False, default=ProbeType.tcp)
-    probe_port: Mapped[int] = mapped_column(Integer, nullable=False, default=80)
-    ping_enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
-    dns_enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
-    dns_expected_ip: Mapped[str | None] = mapped_column(String(255), nullable=True)
-    poll_interval_seconds: Mapped[int | None] = mapped_column(Integer, nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, default=lambda: datetime.now(timezone.utc)
     )