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/audit.py

@@ -22,12 +22,17 @@ async def log_audit(
     entity_id: str | None = None,
     detail: dict | None = None,
 ) -> None:
-    """Write one audit event. Never raises — failures are logged and swallowed."""
+    """Write one audit event, and its timeline entry when the action has one
+    (steward.timeline.catalog.audit_type). Never raises — failures are logged
+    and swallowed."""
+    from steward.core.ids import time_ordered_id
     from steward.models.audit import AuditEvent
+    from steward.timeline.record import from_audit
     try:
         async with app.db_sessionmaker() as db:
             async with db.begin():
-                db.add(AuditEvent(
+                event = AuditEvent(
+                    id=time_ordered_id(),
                     user_id=user_id,
                     username=username or "unknown",
                     action=action,
@@ -35,7 +40,14 @@ async def log_audit(
                     entity_id=entity_id,
                     detail_json=json.dumps(detail) if detail else None,
                     timestamp=datetime.now(timezone.utc),
-                ))
+                )
+                db.add(event)
+                try:
+                    # In a savepoint: an entry that fails leaves the audit row standing.
+                    async with db.begin_nested():
+                        await from_audit(db, event, detail)
+                except Exception:
+                    logger.exception("Audit event %r: its timeline entry could not be written", action)
     except Exception:
         logger.exception("Failed to write audit event action=%r", action)