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/templates/timeline/_entry.html

@@ -0,0 +1,21 @@
+{# One timeline entry's parts, shared by the Timeline page and a host's card.
+   `item` is a timeline.service.view() row. #}
+
+{# What happened: its kind, the sentence, a note, and the full record while it is kept. #}
+{% macro entry_what(item) -%}
+{%- set e = item.entry -%}
+<div>
+  {%- if item.kind_label %}<span class="tone-dim">{{ item.kind_label }} · </span>{% endif -%}
+  {{ e.summary }}
+</div>
+{%- if item.note %}<div class="field-hint">{{ item.note }}</div>{% endif %}
+{%- if item.url %}<div class="field-hint"><a href="{{ item.url }}">Details</a></div>
+{%- elif item.gone %}<div class="field-hint">Details no longer kept</div>{% endif %}
+{%- endmacro %}
+
+{# Where: the hosts it happened on, each linked to its timeline, and what it happened to. #}
+{% macro entry_where(e) -%}
+{%- for h in e.hosts %}<a href="/timeline/?host={{ h|urlencode }}">{{ h }}</a>{{ ", " if not loop.last }}{% endfor -%}
+{%- if e.subject and e.subject not in e.hosts %}{% if e.hosts %}<br>{% endif %}<span class="field-hint">{{ e.subject }}</span>{% endif -%}
+{%- if not e.hosts and not e.subject %}<span class="tone-dim">—</span>{% endif -%}
+{%- endmacro %}