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

tests/integration/test_monitors.py

@@ -0,0 +1,85 @@
+"""Integration: unified Monitor schema + status source against live Postgres.
+
+Validates the 0022 unification migration applied (monitors/monitor_results
+exist, the old per-type tables are gone) and that monitor_status_source rolls
+up MonitorResult history into a StatusEntry. Requires STEWARD_DATABASE_URL.
+"""
+from __future__ import annotations
+
+import asyncio
+import os
+import uuid
+from datetime import datetime, timezone, timedelta
+
+import pytest
+
+pytestmark = pytest.mark.integration
+
+_NEEDS_DB = pytest.mark.skipif(
+    not os.environ.get("STEWARD_DATABASE_URL"),
+    reason="integration test needs a live Postgres (STEWARD_DATABASE_URL)",
+)
+
+
+@pytest.fixture
+def app():
+    if not os.environ.get("STEWARD_DATABASE_URL"):
+        pytest.skip("needs Postgres")
+    from steward.app import create_app
+    return create_app(testing=False)
+
+
+@_NEEDS_DB
+def test_old_monitor_tables_dropped(app):
+    from sqlalchemy import text
+
+    async def _go():
+        async with app.db_sessionmaker() as s:
+            # New tables exist…
+            await s.execute(text("SELECT COUNT(*) FROM monitors"))
+            await s.execute(text("SELECT COUNT(*) FROM monitor_results"))
+            # …old ones are gone (regclass of a missing table is NULL).
+            for tbl in ("ping_results", "dns_results", "http_monitors", "http_results"):
+                missing = (await s.execute(
+                    text("SELECT to_regclass(:t)"), {"t": tbl})).scalar()
+                assert missing is None, f"{tbl} should have been dropped"
+
+    asyncio.run(_go())
+
+
+@_NEEDS_DB
+def test_status_source_rolls_up_results(app):
+    from sqlalchemy import text
+    from steward.models.monitors import Monitor, MonitorResult
+    from steward.core.status import monitor_status_source
+
+    mid = str(uuid.uuid4())
+    now = datetime.now(timezone.utc)
+
+    async def _go():
+        async with app.db_sessionmaker() as s:
+            async with s.begin():
+                # Clean slate for a deterministic assertion.
+                await s.execute(text("DELETE FROM monitor_results"))
+                await s.execute(text("DELETE FROM monitors"))
+                s.add(Monitor(id=mid, name="custom-http", type="http",
+                              target="https://example.test", host_id=None,
+                              config_json="{}", enabled=True))
+            async with s.begin():
+                for i, up in enumerate([True, True, False, True]):
+                    s.add(MonitorResult(
+                        id=str(uuid.uuid4()), monitor_id=mid,
+                        checked_at=now - timedelta(minutes=4 - i),
+                        is_up=up, response_ms=10.0 if up else None,
+                    ))
+            entries = await monitor_status_source(s)
+        return entries
+
+    entries = asyncio.run(_go())
+    assert len(entries) == 1
+    e = entries[0]
+    assert e.kind == "http" and e.name == "custom-http"
+    assert e.target == "https://example.test"
+    assert e.status == "up"                 # latest result is up
+    assert len(e.heartbeat) == 4
+    assert e.uptime["24h"] == 75.0          # 3 of 4 up