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/reports.py

@@ -12,9 +12,8 @@ from email.message import EmailMessage
 from sqlalchemy import and_, case, func, select
 
 from steward.models.alerts import AlertRule, AlertState, AlertStateEnum, AlertEvent
-from steward.models.hosts import Host
 from steward.models.metrics import PluginMetric
-from steward.models.monitors import PingResult, PingStatus
+from steward.models.monitors import Monitor, MonitorResult
 
 logger = logging.getLogger(__name__)
 
@@ -28,32 +27,28 @@ async def build_report_data(app) -> dict:
     cutoff_24h = now - timedelta(hours=24)
 
     async with app.db_sessionmaker() as db:
-        # ── Uptime summary (7d) ───────────────────────────────────────────────
-        host_result = await db.execute(
-            select(Host).where(Host.ping_enabled.is_(True)).order_by(Host.name)
-        )
-        hosts = host_result.scalars().all()
+        # ── Uptime summary (7d) — per enabled monitor of any type ─────────────
+        monitors = (await db.execute(
+            select(Monitor).where(Monitor.enabled.is_(True)).order_by(Monitor.name)
+        )).scalars().all()
 
         uptime_rows = await db.execute(
             select(
-                PingResult.host_id,
-                func.count(PingResult.id).label("total"),
-                func.sum(case((PingResult.status == PingStatus.up, 1), else_=0)).label("up"),
+                MonitorResult.monitor_id,
+                func.count(MonitorResult.id).label("total"),
+                func.sum(case((MonitorResult.is_up.is_(True), 1), else_=0)).label("up"),
             )
-            .where(PingResult.probed_at >= cutoff_7d)
-            .group_by(PingResult.host_id)
+            .where(MonitorResult.checked_at >= cutoff_7d)
+            .group_by(MonitorResult.monitor_id)
         )
         uptime_map: dict[str, float | None] = {}
         for row in uptime_rows:
             pct = round(float(row.up) / float(row.total) * 100, 2) if row.total else None
-            uptime_map[row.host_id] = pct
+            uptime_map[row.monitor_id] = pct
 
-        uptime_summary = []
-        for h in hosts:
-            uptime_summary.append({
-                "name": h.name,
-                "pct_7d": uptime_map.get(h.id),
-            })
+        uptime_summary = [
+            {"name": m.name, "pct_7d": uptime_map.get(m.id)} for m in monitors
+        ]
 
         # ── Active alerts ─────────────────────────────────────────────────────
         active_result = await db.execute(
@@ -112,27 +107,27 @@ async def build_report_data(app) -> dict:
             for row in top_routers_result
         ]
 
-        # ── Hosts currently down ──────────────────────────────────────────────
-        latest_ping_subq = (
-            select(PingResult.host_id, func.max(PingResult.probed_at).label("max_at"))
-            .group_by(PingResult.host_id)
+        # ── Monitors currently down (latest result is_up = false) ─────────────
+        latest_subq = (
+            select(MonitorResult.monitor_id, func.max(MonitorResult.checked_at).label("max_at"))
+            .group_by(MonitorResult.monitor_id)
             .subquery()
         )
         down_result = await db.execute(
-            select(Host.name)
-            .join(PingResult, PingResult.host_id == Host.id)
+            select(Monitor.name)
+            .join(MonitorResult, MonitorResult.monitor_id == Monitor.id)
             .join(
-                latest_ping_subq,
+                latest_subq,
                 and_(
-                    PingResult.host_id == latest_ping_subq.c.host_id,
-                    PingResult.probed_at == latest_ping_subq.c.max_at,
+                    MonitorResult.monitor_id == latest_subq.c.monitor_id,
+                    MonitorResult.checked_at == latest_subq.c.max_at,
                 ),
             )
             .where(
-                Host.ping_enabled.is_(True),
-                PingResult.status == PingStatus.down,
+                Monitor.enabled.is_(True),
+                MonitorResult.is_up.is_(False),
             )
-            .order_by(Host.name)
+            .order_by(Monitor.name)
         )
         hosts_down = [row.name for row in down_result]
 
@@ -157,7 +152,7 @@ def _render_report_text(data: dict) -> str:
     # ── Hosts currently down ──────────────────────────────────────────────────
     down = data["hosts_down"]
     if down:
-        lines.append(f"⚠  {len(down)} host(s) currently DOWN: {', '.join(down)}")
+        lines.append(f"⚠  {len(down)} monitor(s) currently DOWN: {', '.join(down)}")
         lines.append("")
 
     # ── Uptime summary ────────────────────────────────────────────────────────
@@ -168,7 +163,7 @@ def _render_report_text(data: dict) -> str:
         pct_str = f"{pct:.2f}%" if pct is not None else "no data"
         lines.append(f"  {entry['name']:<32} {pct_str}")
     if not data["uptime_summary"]:
-        lines.append("  (no ping-enabled hosts)")
+        lines.append("  (no monitors configured)")
     lines.append("")
 
     # ── Active alerts ─────────────────────────────────────────────────────────