feat: add quota-triggered per-user mailbox cleanup

Inflate the Dovecot-visible quota to 140% of the configured
max_mailbox_size so that Delta Chat clients (which warn at
80% of IMAP-reported quota) never show quota warnings.
A quota_warning at 72% of the inflated limit triggers
chatmail-quota-expire, which trims the mailbox to 80% of the
configured limit.  Existing over-quota mailboxes start
receiving mail again immediately after deploy
without any manual operator action needed.

doc/source/overview.rst

@@ -102,8 +102,15 @@ short overview of ``chatmaild`` services:
    Apple/Google/Huawei.
 
 -  `chatmail-expire <https://github.com/chatmail/relay/blob/main/chatmaild/src/chatmaild/expire.py>`_
-   deletes users if they have not logged in for a longer while.
-   The timeframe can be configured in ``chatmail.ini``.
+   deletes old messages, large messages, and entire mailboxes
+   of users who have not logged in for longer than
+   ``delete_inactive_users_after`` days.
+
+-  ``chatmail-quota-expire``
+   is called by Dovecot's ``quota_warning`` mechanism when a
+   mailbox exceeds ``max_mailbox_size``.
+   It removes the oldest messages
+   until usage drops to a safe level.
 
 -  `lastlogin <https://github.com/chatmail/relay/blob/main/chatmaild/src/chatmaild/lastlogin.py>`_
    is contacted by Dovecot when a user logs in and stores the date of
@@ -156,6 +163,8 @@ Chatmail relay dependency diagram
         /home/vmail/.../user"];
         dovecot --- |lastlogin.socket|lastlogin;
         dovecot --- chatmail-metadata;
+        dovecot --- |quota-warning|chatmail-quota-expire;
+        chatmail-quota-expire --- maildir;
         lastlogin --- maildir;
         doveauth --- maildir;
         chatmail-expire-daily --- maildir;