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.

CHANGELOG.md

@@ -1,5 +1,20 @@
 # Changelog for chatmail deployment 
 
+## Unreleased
+
+### Features
+
+- Add per-user quota-triggered cleanup (`chatmail-quota-expire`).
+  When a mailbox exceeds the configured ``max_mailbox_size``,
+  Dovecot runs the new script which removes the oldest
+  messages until usage drops to a safe level.
+  No operator action is required after upgrading;
+  existing over-quota mailboxes start receiving mail
+  again immediately and are cleaned up automatically.
+  The daily `chatmail-expire` timer continues to handle
+  deletion of old messages, large messages,
+  and inactive user mailboxes.
+
 ## 1.9.0 2025-12-18
 
 ### Documentation