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>

app/db.py

@@ -70,6 +70,28 @@ CREATE TABLE IF NOT EXISTS articles (
 
 CREATE INDEX IF NOT EXISTS idx_articles_ticket ON articles(ticket_id, created_at);
 
+CREATE TABLE IF NOT EXISTS versions (
+    source       TEXT PRIMARY KEY,
+    version      TEXT,
+    released_at  TEXT,
+    checked_at   TEXT,
+    updated_at   TEXT
+);
+
+CREATE TABLE IF NOT EXISTS downloads (
+    id              INTEGER PRIMARY KEY AUTOINCREMENT,
+    version         TEXT NOT NULL,
+    filename        TEXT NOT NULL,
+    url             TEXT,
+    size            INTEGER,
+    sha256          TEXT,
+    remote_modified TEXT,
+    downloaded_at   TEXT,
+    UNIQUE(version, filename)
+);
+
+CREATE INDEX IF NOT EXISTS idx_downloads_version ON downloads(version);
+
 CREATE TABLE IF NOT EXISTS sync_runs (
     id           INTEGER PRIMARY KEY AUTOINCREMENT,
     started_at   TEXT,