feat(timeline): one record of what happened to the infrastructure, newest first (#6348)

A stored log, timeline_entries, with one row per thing that happened. Each
source writes its row in the same transaction as its own record, so the
history outlives the 30/90/365-day retention of the tables it points to:

- alert events (core/alerts._add_event): fired, resolved, acknowledged,
  went silent, readings back;
- finished Ansible runs (executor, and the startup sweep of interrupted
  runs): changed something, changed nothing, failed, dry run;
- audit rows (core/audit.log_audit): Steward's settings, users and access,
  sign-ins, backups. Actions another writer records are left to it.

Entries carry a kind (fixed) and an area (Service, Host OS, Hardware,
Storage, Network, Security, Monitoring). Settings → Timeline edits the
areas and each type's area and visibility; both are applied when the
timeline is read, so a change re-sorts past entries and a hidden type
comes back when shown. Kept forever unless a retention is set.

The first start fills the timeline from the last 90 days of records once,
skipping any already covered. A host rename carries its entries.

UI: Timeline in the sidebar (filter by host, area, kind; scrolls on), a
History card on each host page. MCP: get_timeline.

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

steward/core/cleanup.py

@@ -9,6 +9,7 @@ from typing import TYPE_CHECKING
 from sqlalchemy import delete
 
 from steward.core.anomalies import prune as prune_anomalies
+from steward.timeline.service import prune as prune_timeline
 from steward.models.alerts import AlertEvent
 from steward.models.ansible import AnsibleRun
 from steward.models.audit import AuditEvent
@@ -81,6 +82,10 @@ async def run_cleanup(app: "Quart") -> dict:
             pruned = await prune_anomalies(session, cutoff)
             removed.append([f"metric_anomalies: reviewed, ended over {data_days} days ago",
                             pruned])
+            # The timeline keeps its own window, everything by default.
+            pruned, days = await prune_timeline(session, now)
+            if days:
+                removed.append([f"timeline_entries: older than {days} days", pruned])
 
         files = await asyncio.to_thread(prune_run_logs, ARTIFACT_DIR, cutoff)
         removed.append([f"Ansible run log files: older than {data_days} days", files])