feat(monitors): a monitor's page reads as a page — response time over 24h, labelled actions, how long, not "ago"; the add form says what each type checks (#5214)

Monitor page: a response-time chart for the last day with failed checks
marked; Edit / Pause / Delete as labelled buttons; the state tile says
"for 29m" instead of "for at least 29m 25s ago"; the check strip fills
its card; no failures is one line, not an empty box; Settings names the
type, target and the actual global interval, with readable field names.

Add monitor: a line under the fields says what the selected type checks
and when it counts as up; Name says it defaults to the target.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/monitors/service.py

@@ -8,7 +8,7 @@ from __future__ import annotations
 
 import json
 from collections.abc import Mapping
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
 
 from sqlalchemy import func, select
 
@@ -255,6 +255,26 @@ async def monitor_states(db, monitors: list[Monitor]) -> dict[str, dict]:
     return out
 
 
+async def response_history(db, monitor_id: str, now: datetime, hours: int = 24,
+                           points: int = 96) -> list[dict]:
+    """The monitor's response time and failed checks over the last `hours`,
+    in about `points` buckets, oldest first: [{at (epoch ms), ms, down}].
+    `ms` is the bucket's average (None when no check returned a time, as for
+    DNS); `down` counts the failed checks in it."""
+    width = max(60, hours * 3600 // points)
+    bucket = func.date_bin(func.make_interval(0, 0, 0, 0, 0, 0, width), MonitorResult.checked_at,
+                           func.to_timestamp(0)).label("bucket")
+    rows = (await db.execute(
+        select(bucket, func.avg(MonitorResult.response_ms),
+               func.count().filter(MonitorResult.is_up.is_(False)))
+        .where(MonitorResult.monitor_id == monitor_id,
+               MonitorResult.checked_at >= now - timedelta(hours=hours))
+        .group_by(bucket).order_by(bucket)
+    )).all()
+    return [{"at": int(at.timestamp() * 1000), "ms": None if ms is None else round(float(ms), 1),
+             "down": int(down)} for at, ms, down in rows]
+
+
 async def recent_failures(db, monitor_id: str, limit: int = 10) -> list[MonitorResult]:
     """The monitor's latest failed checks, newest first."""
     return list((await db.execute(