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/_host_card.html

@@ -0,0 +1,25 @@
+{# A host's newest timeline entries, loaded into its page: what happened to it
+   lately, with the whole of it a click away. #}
+{% from "_macros.html" import empty_state %}
+{% from "timeline/_entry.html" import entry_what %}
+<section class="card" aria-labelledby="host-history">
+  <div style="display:flex;align-items:baseline;justify-content:space-between;gap:1rem;">
+    <h3 id="host-history" class="section-title">History</h3>
+    <a class="field-hint" href="/timeline/?host={{ host.name|urlencode }}">All of {{ host.name }}'s history</a>
+  </div>
+  {% if items %}
+  <table class="table">
+    <tbody>
+      {% for item in items %}
+      <tr>
+        <td class="nowrap" style="width:1%;">{{ item.entry.at|when }}</td>
+        <td>{{ entry_what(item) }}</td>
+        <td class="nowrap" style="width:1%;">{{ item.entry.actor }}</td>
+      </tr>
+      {% endfor %}
+    </tbody>
+  </table>
+  {% else %}
+  {{ empty_state("Nothing has happened to " ~ host.name ~ " that the timeline records yet.") }}
+  {% endif %}
+</section>