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/mcp/tools/__init__.py

@@ -12,9 +12,10 @@ each argument's empty value means, and what comes back.
 """
 from __future__ import annotations
 
-from . import alerts, anomalies, audit, automation, backups, hosts, metrics, monitors, overview, session
+from . import (alerts, anomalies, audit, automation, backups, hosts, metrics, monitors, overview, session,
+               timeline)
 
-_MODULES = (session, overview, hosts, monitors, alerts, anomalies, metrics, audit, automation,
+_MODULES = (session, overview, hosts, monitors, alerts, anomalies, metrics, audit, timeline, automation,
             backups)