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.

cmdeploy/src/cmdeploy/dovecot/dovecot.conf.j2

@@ -149,12 +149,26 @@ plugin {
 }
 
 plugin {
-  # for now we define static quota-rules for all users 
   quota = maildir:User quota
-  quota_rule = *:storage={{ config.max_mailbox_size }}
   quota_max_mail_size={{ config.max_message_size }}
   quota_grace = 0
-  # quota_over_flag_value = TRUE
+
+  # Inflate the dovecot-visible quota so that Delta Chat clients
+  # (which warn at 80% of the IMAP-reported limit) never see
+  # quota warnings -- expire kicks in well before that point.
+  quota_rule = *:storage={{ config.max_mailbox_size_mb * 140 // 100 }}M
+
+  # Trigger when usage reaches the configured max_mailbox_size
+  # (72% of inflated = ~100% of configured), then expire oldest
+  # messages down to 80% of the configured max_mailbox_size.
+  quota_warning = storage=72%% quota-warning {{ config.max_mailbox_size_mb * 80 // 100 }} {{ config.mailboxes_dir }}/%u
+}
+
+service quota-warning {
+  executable = script /usr/local/lib/chatmaild/venv/bin/chatmail-quota-expire
+  user = vmail
+  unix_listener quota-warning {
+  }
 }
 
 # push_notification configuration

cmdeploy/src/cmdeploy/mtail/delivered_mail.mtail

@@ -78,3 +78,11 @@ counter rejected_unencrypted_mail_count
 /Rejected unencrypted mail/ {
   rejected_unencrypted_mail_count++
 }
+
+counter quota_expire_runs
+counter quota_expire_removed_files
+
+/quota-expire: removed (?P<count>\d+) message\(s\)/ {
+  quota_expire_runs++
+  quota_expire_removed_files += $count
+}

cmdeploy/src/cmdeploy/tests/online/test_2_deltachat.py

@@ -88,9 +88,12 @@ class TestEndToEndDeltaChat:
             return int(float(number) * units[unit])
 
         quota = parse_size_limit(chatmail_config.max_mailbox_size)
+        # Dovecot quota is inflated to 140% of the configured limit
+        # so that quota-expire keeps users below the warning threshold.
+        dovecot_quota = quota * 140 // 100
 
         lp.sec(f"filling remote inbox for {user}")
-        fn = f"7743102289.M843172P2484002.c20,S={quota},W=2398:2,"
+        fn = f"7743102289.M843172P2484002.c20,S={dovecot_quota},W=2398:2,"
         path = chatmail_config.mailboxes_dir.joinpath(user, "cur", fn)
         sshexec = get_sshexec(sshdomain)
         sshexec(call=rshell.write_numbytes, kwargs=dict(path=str(path), num=120))