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/alerts/service.py

@@ -833,10 +833,11 @@ class PartOfCoveringRule(ValueError):
                          f"rule per host; change or disable it there.")
 
 
-async def acknowledge(db, rule_id: str, user_id: str | None) -> AlertState:
+async def acknowledge(db, rule_id: str, user_id: str | None, actor: str | None = None) -> AlertState:
     """Acknowledge a firing alert, in the caller's transaction, and record it
     in the alert's history as a firing → acknowledged event by `user_id`, so
-    who saw it outlives the alert (#5463).
+    who saw it outlives the alert (#5463). `actor` names them on the timeline
+    when it isn't their username alone ("<user> via MCP").
 
     LookupError when the rule has no state; ValueError when it is not firing.
     """
@@ -853,8 +854,9 @@ async def acknowledge(db, rule_id: str, user_id: str | None) -> AlertState:
     state.acknowledged_by = user_id
     state.acknowledged_at = now
     state.last_transition_at = now
-    _add_event(db, rule_id, AlertStateEnum.firing.value, AlertStateEnum.acknowledged.value,
-               None, now, actor_id=user_id)
+    rule = await db.get(AlertRule, rule_id)
+    await _add_event(db, rule, AlertStateEnum.firing.value, AlertStateEnum.acknowledged.value,
+                     None, now, actor_id=user_id, actor=actor)
     return state