feat: serve an APPVERSIONS.json index file to clients via IMAP metadata

This is designed to help implement self-updating APKs (and later other clients),
see counterpart https://github.com/chatmail/core/pull/8557

doc/source/overview.rst

@@ -249,6 +249,28 @@ Fresh chatmail addresses have a mailbox directory that contains:
    directories will typically be empty unless the user of that address
    hasn’t been online for a while.
 
+App version information (experimental)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A chatmail relay ships the repository's
+`APPVERSIONS.json <https://github.com/chatmail/relay/blob/main/APPVERSIONS.json>`_
+and serves its content under the IMAP METADATA key
+``/shared/vendor/deltachat/appversions``.
+Chat apps installed outside of app stores read this key
+to learn about updates and where to download them.
+The mechanism is experimental and may change.
+
+The file travels with the normal deploy:
+update the repository checkout and run ``cmdeploy run``.
+Local modifications of ``APPVERSIONS.json`` are deployed as-is,
+so you can serve your own app version information,
+including links to app downloads.
+
+Note that as of August 2026, only Delta Chat Android is beginning
+to support discovering app versions from relays.
+Consumers of relay-provided app version information
+need to verify themselves that downloaded app files are valid.
+
 Active ports
 ~~~~~~~~~~~~