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/settings.py

@@ -52,6 +52,17 @@ DEFAULTS: dict[str, Any] = {
     "data.retention_days": 90,
     # The audit log is a security record: kept longer than operational data.
     "audit.retention_days": 365,
+    # The timeline is the record of what happened to the infrastructure: kept
+    # until this many days are set (0 keeps it all). Its areas, and each entry
+    # type's area and visibility, are Settings → Timeline (steward/timeline).
+    "timeline.retention_days": 0,
+    "timeline.areas": [["service", "Service"], ["host_os", "Host OS"], ["hardware", "Hardware"],
+                       ["storage", "Storage"], ["network", "Network"], ["security", "Security"],
+                       ["monitoring", "Monitoring"]],
+    "timeline.types": {},
+    "timeline.area_moved": {},
+    # When the timeline was first filled from the records that predate it.
+    "timeline.backfilled_at": "",
     "monitors.poll_interval_seconds": 60,
     # Root log level: DEBUG, INFO, WARNING or ERROR (steward/core/logging_setup.py).
     # WARNING also hides the per-request access lines.