feat(ui): audit log, alert history and Ansible runs load as you scroll (#5211)

#5211 "Lists load as you scroll: audit, alert history, Ansible runs"
(milestone 477), per preference 172.

- core/paging.py: keyset paging over (time, id), newest first, with an
  opaque "<iso>|<id>" cursor; a page reads one row past itself to know
  whether another follows. No OFFSET, so rows added while someone scrolls
  never shift the pages after them.
- _macros.html scroll_more / scroll_end and static/scroll-more.js, written
  once: an IntersectionObserver with a 600px margin fetches the next page
  before the reader reaches the end; "Loading more…" while in flight,
  "End of the list." at the end, and a failed page says so with a Retry
  button instead of re-firing on every scroll.
- Audit log: no "Load more" link and no 500-row cap.
- Alert history: no 200-row stop with "pick a shorter range";
  alert_svc.history takes the cursor, so the MCP tool and the page share
  one query.
- Ansible runs: no silent cap of 50.

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

tests/integration/test_alert_history_page.py

@@ -1,7 +1,7 @@
 """Integration: /alerts/history (#5066) against real Postgres. A viewer can
 read it; it shows the transition, value against the condition, delivery
 outcome with its error, and a maintenance mute; ?rule= narrows it to one
-rule, and an HTMX request gets only the rows."""
+rule, an HTMX request gets only the rows, and ?before= the next page."""
 from __future__ import annotations
 
 import asyncio
@@ -77,6 +77,16 @@ def test_a_viewer_reads_the_alert_history(app):
         assert "<html" not in rows and "<table" in rows
         assert f"old-{tag}" in rows and f"cold-note-{tag}" not in rows
 
+        # The next page as the reader scrolls: only older rows, without the table
+        # around them, and the list says where it ends (#5211).
+        from steward.core.paging import cursor
+        resp = await client.get("/alerts/history", query_string={
+            "range": "24h", "rule": hot_id, "before": cursor(now - timedelta(minutes=3), "")})
+        older = await resp.get_data(as_text=True)
+        assert resp.status_code == 200 and "<table" not in older
+        assert "cpu_pct = 97.5" in older and f"old-{tag}" in older and f"patch-{tag}" not in older
+        assert "End of the list." in older and "data-scroll-more" not in older
+
         # The rules list links each rule to its history.
         assert f'/alerts/history?rule={hot_id}' in await (await client.get("/alerts/")).get_data(as_text=True)