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/base.html

@@ -13,6 +13,7 @@
       <a href="{{ url_for('index') }}">Übersicht</a>
       <a href="{{ url_for('analytics_view') }}">Auswertung</a>
       <a href="{{ url_for('changes') }}">Änderungen</a>
+      <a href="{{ url_for('downloads_view') }}">Downloads</a>
       <a href="{{ url_for('new_ticket_form') }}">+ Neues Ticket</a>
       <a href="{{ zammad_url }}" target="_blank" rel="noopener">Zammad&nbsp;↗</a>
     </nav>