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/list.html

@@ -0,0 +1,54 @@
+{# timeline/list.html — everything that happened to the infrastructure, newest
+   first, one list: alerts, playbook runs, changes to Steward's settings.
+   Filters are by host, area (what was affected) and kind (how it came about);
+   Settings → Timeline decides each type's area and whether it is shown, and
+   "Include hidden" shows the types it hides. #}
+{% extends "base.html" %}
+{% from "_macros.html" import page_header, empty_state %}
+{% block title %}Timeline — Steward{% endblock %}
+{% block content %}
+{% call page_header("Timeline", "What happened, newest first: alerts, playbook runs and changes, on every host.") %}
+  {% if session.user_role == 'admin' %}<a class="btn btn-ghost btn-sm" href="/settings/timeline/">Areas and types</a>{% endif %}
+{% endcall %}
+
+<form class="filter-bar" method="get" action="/timeline/" role="search">
+  <select name="host" aria-label="Host" onchange="this.form.submit()">
+    <option value="">All hosts</option>
+    {% for h in hosts %}<option value="{{ h }}"{% if filters.host == h %} selected{% endif %}>{{ h }}</option>{% endfor %}
+  </select>
+  <select name="area" aria-label="Area" onchange="this.form.submit()">
+    <option value="">All areas</option>
+    {% for key, label in areas %}<option value="{{ key }}"{% if filters.area == key %} selected{% endif %}>{{ label }}</option>{% endfor %}
+  </select>
+  <select name="kind" aria-label="Kind" onchange="this.form.submit()">
+    <option value="">All kinds</option>
+    {% for key, label in kinds.items() %}<option value="{{ key }}"{% if filters.kind == key %} selected{% endif %}>{{ label }}</option>{% endfor %}
+  </select>
+  <label class="field-hint" style="display:flex;align-items:center;gap:0.35rem;">
+    <input type="checkbox" name="hidden" value="1"{% if filters.hidden %} checked{% endif %} onchange="this.form.submit()">
+    Include hidden types
+  </label>
+  <noscript><button type="submit" class="btn btn-ghost btn-sm">Filter</button></noscript>
+</form>
+
+{% if items %}
+<div class="card-flush">
+  <table class="table">
+    <thead>
+      <tr>
+        <th>When ({{ tz_label() }})</th>
+        <th>What happened</th>
+        <th>Where</th>
+        <th>Who</th>
+        <th>Area</th>
+      </tr>
+    </thead>
+    <tbody>
+      {% include "timeline/_rows.html" %}
+    </tbody>
+  </table>
+</div>
+{% else %}
+<div class="card">{{ empty_state("Nothing on the timeline matches these filters." if keep else "Nothing has been recorded on the timeline yet.") }}</div>
+{% endif %}
+{% endblock %}