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/backup/contents.py

@@ -35,6 +35,8 @@ KEPT: frozenset[str] = frozenset({
 EXCLUDED: frozenset[str] = frozenset({
     # Logs and history (the 2026-10-15 ruling): a recovery doesn't need them.
     "audit_events", "alert_events",
+    # The timeline of what happened to the infrastructure: history.
+    "timeline_entries",
     # Live state: each rule starts inactive, and alert_reconcile gives a
     # restored rule its state row within a minute (ensure_rule_states).
     "alert_states",