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>

steward/alerts/service.py

@@ -241,17 +241,18 @@ async def list_rules(db) -> list[AlertRule]:
 
 
 async def history(db, since: datetime, rule_id: str | None = None,
-                  limit: int = 200) -> list[tuple[AlertEvent, AlertRule]]:
+                  limit: int = 200, before: str | None = None) -> list[tuple[AlertEvent, AlertRule]]:
     """Alert transitions since `since`, newest first, with their rule; one
-    rule's when `rule_id` is given. Returns up to limit+1 rows so a caller can
-    tell the list was cut."""
+    rule's when `rule_id` is given; only those older than the `before` cursor
+    when given (core.paging). Returns up to limit+1 rows so a caller can tell
+    the list was cut."""
+    from steward.core.paging import keyset
     stmt = (select(AlertEvent, AlertRule)
             .join(AlertRule, AlertEvent.rule_id == AlertRule.id)
-            .where(AlertEvent.transitioned_at >= since)
-            .order_by(AlertEvent.transitioned_at.desc())
-            .limit(limit + 1))
+            .where(AlertEvent.transitioned_at >= since))
     if rule_id:
         stmt = stmt.where(AlertEvent.rule_id == rule_id)
+    stmt = keyset(stmt, AlertEvent.transitioned_at, AlertEvent.id, before, limit)
     return [tuple(row) for row in (await db.execute(stmt)).all()]