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>

.env.example

@@ -21,6 +21,21 @@ ZAMMAD_PASSWORD=
 # Intervall des Hintergrund-Abrufs in Sekunden
 POLL_INTERVAL_SECONDS=180
 
+# --- EPIRent-Versionsüberwachung ----------------------------
+# Liest "Aktuelle Version / NNNNN vom TT.MM.JJJJ" von der Download-Seite und
+# meldet Änderungen wie jede andere Änderung (Feed + E-Mail).
+EPIRENT_CHECK_ENABLED=true
+EPIRENT_URL=https://www.epi.rent/downloads
+EPIRENT_CHECK_INTERVAL_SECONDS=1800
+
+# Download-Archiv: sichert die Pakete je Versionsnummer (Verlauf/Cache).
+# ACHTUNG: ca. 1,1 GB pro Versionsstand (Server ~923 MB + Client ~218 MB).
+EPIRENT_DOWNLOAD_ENABLED=true
+EPIRENT_DOWNLOAD_URLS=https://download.epi.rent/latest/epirentServer.zip,https://download.epi.rent/latest/epirentClient.zip
+# Wie viele Versionsstände aufbewahrt werden (3 => ca. 3,4 GB).
+# 0 = unbegrenzt (nichts wird automatisch gelöscht – Plattenplatz im Auge behalten!)
+EPIRENT_DOWNLOAD_KEEP=3
+
 # --- Web ----------------------------------------------------
 # Pfad-Präfix hinter Apache (srhnweb/episupport)
 EPISUPPORT_URL_PREFIX=/episupport