Watch EPIRent version and archive its download packages

- Poll the public download page for "Aktuelle Version <nr> vom <date>",
  report changes through the normal change feed and notification mail,
  and show the current version as a card on the overview. Runs on its own
  interval (default 30 min); the first run only records a baseline.
- Archive epirentServer.zip / epirentClient.zip per version number so we
  keep a download cache and history: streamed to disk with SHA-256 and
  size, fetched in a background thread so ticket polling is unaffected,
  with a free-space check before downloading (~1.1 GB per version).
- Retention is configurable via EPIRENT_DOWNLOAD_KEEP, where 0 means
  unlimited; a successful archive is reported as a change and by mail.
- New Downloads page listing the archived versions with checksums and
  links to fetch the cached files.
- Notification lines now omit the arrow when there is no previous value
  and drop the "#" for entries without a ticket.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/downloads.html

@@ -0,0 +1,41 @@
+{% extends "base.html" %}
+{% block title %}Downloads · EPI-Support{% endblock %}
+{% block content %}
+
+<section class="panel">
+  <div class="panel-head">
+    <h2>EPIRent Download-Archiv</h2>
+    {% if current %}
+      <span class="hint-inline">aktuelle Version: <b>{{ current.version }}</b> vom {{ current.released_at }}</span>
+    {% endif %}
+  </div>
+
+  {% if versions %}
+    {% for v in versions %}
+      <div class="dl-version">
+        <div class="dl-head">
+          <span class="dl-ver">Version {{ v.version }}</span>
+          <span class="dl-meta">{{ human(v.total) }} · gesichert am {{ v.downloaded_at|dt }}</span>
+        </div>
+        <ul class="dl-files">
+          {% for f in v.files %}
+            <li>
+              <a href="{{ url_for('download_file', version=v.version, filename=f.filename) }}">⬇ {{ f.filename }}</a>
+              <span class="dl-size">{{ human(f.size) }}</span>
+              {% if f.sha256 %}<code class="dl-hash" title="SHA-256">{{ f.sha256[:16] }}…</code>{% endif %}
+            </li>
+          {% endfor %}
+        </ul>
+      </div>
+    {% endfor %}
+    {% if keep and keep > 0 %}
+      <p class="hint">Es werden die letzten <b>{{ keep }}</b> Versionsstände aufbewahrt; ältere werden automatisch gelöscht.</p>
+    {% else %}
+      <p class="hint">Aufbewahrung: <b>unbegrenzt</b> – es wird nichts automatisch gelöscht (ca. 1,1 GB je Stand).</p>
+    {% endif %}
+  {% else %}
+    <p class="muted">Noch keine Pakete archiviert. Der Poller lädt sie im Hintergrund,
+      sobald eine Version bekannt ist (ca. 1,1 GB pro Stand – das dauert einen Moment).</p>
+  {% endif %}
+</section>
+{% endblock %}