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/_macros.html

@@ -2,9 +2,11 @@
   <li class="{{ 'unseen' if not c.seen else '' }}"
       {% if c.detail %}title="{{ c.detail }}"{% endif %}>
     <span class="tag tag-{{ c.field }}">{{ c.label }}</span>
-    <a class="cnum" href="{{ url_for('ticket_detail', ticket_id=c.ticket_id) }}">#{{ c.ticket_number }}</a>
+    {% if c.ticket_id %}
+      <a class="cnum" href="{{ url_for('ticket_detail', ticket_id=c.ticket_id) }}">#{{ c.ticket_number }}</a>
+    {% endif %}
     <span class="ctitle">{{ c.ticket_title }}</span>
-    <span class="cchg">{{ c.old_value or '–' }} → <b>{{ c.new_value or '–' }}</b></span>
+    <span class="cchg">{% if c.old_value %}{{ c.old_value }} → {% endif %}<b>{{ c.new_value or '–' }}</b></span>
     <time>{{ c.detected_at|dt }}</time>
     {% if not c.seen %}
       <form method="post" action="{{ url_for('mark_one_read', change_id=c.id) }}" class="mark-one">