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/index.html

@@ -9,6 +9,12 @@
   <div class="card {{ 'alert' if stats.unseen else '' }}">
     <span class="num">{{ stats.unseen }}</span><span class="lbl">ungelesene Änderungen</span>
   </div>
+  {% if epirent_version %}
+  <div class="card">
+    <span class="num">{{ epirent_version.version }}</span>
+    <span class="lbl">EPIRent-Version<br>vom {{ epirent_version.released_at }}</span>
+  </div>
+  {% endif %}
   <div class="card status">
     {% if last_run %}
       <span class="num {{ 'ok' if last_run.status == 'ok' else 'err' }}">