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/alerts.py

@@ -11,7 +11,7 @@ from steward.mcp._context import current_caller, mcp_app
 from steward.models.alerts import AlertOperator, AlertRule, AlertState, MaintenanceWindow
 from steward.models.users import UserRole
 
-from ._audit import audit_mcp
+from ._audit import audit_mcp, mcp_username
 from ._shape import (
     age_s, bounded, clamp_limit, db_session, iso, lookback, now_utc, pick, rnd,
 )
@@ -389,7 +389,8 @@ async def acknowledge_alert(rule: str) -> dict:
     async with db_session() as db, db.begin():
         r = await _rule(db, rule)
         try:
-            state = await alert_svc.acknowledge(db, r.id, caller.user_id)
+            state = await alert_svc.acknowledge(db, r.id, caller.user_id,
+                                                mcp_username(caller.username))
         except LookupError:
             raise ValueError(f"{r.name} has never been evaluated, so there is nothing to "
                              "acknowledge.") from None