feat: implement B.2 — rotate mail.log by rename + postfix reload

Replaces copytruncate with rename + `postfix reload` (the same mechanism
`postfix logrotate` itself uses), closing the up-to-one-second window where
copytruncate could drop in-flight delivery lines and leave a send-log row
stuck at "queued" forever.

logrotate-mail.conf keeps `create 0644 root root` rather than `nocreate` as
originally planned: verified on a live container that Postfix recreates the
file itself only lazily, on the next write after reload, and at mode 0600 —
unreadable by the unprivileged panel process. `create` hands the file back at
0644 immediately after rename, before Postfix ever touches it.

logtail.follow() re-drains the old file descriptor once more right before
switching to the rotated file, closing the residual gap between the last
poll's drain and the rotation check. readLogTail() treats a momentarily
missing mail.log as an empty screen rather than a logged error.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

build/logrotate-loop.sh

@@ -1,10 +1,13 @@
 #!/bin/sh
-# Periodic logrotate for /var/log/mail.log (spec 9, 10). Postfix's maillog_file
-# is written by postlogd, which keeps the file open for the life of the
-# process — there is no daemon to signal on rotation, so the logrotate.d config
-# uses copytruncate (a brief truncation race can drop the last few in-flight
-# lines, which is an acceptable trade for not having to reload Postfix on every
-# rotation).
+# Periodic logrotate for /var/log/mail.log (spec 9, 10). Rotation renames the
+# file, recreates it (`create 0644 root root`, matching a cold container
+# start), then runs `postfix reload` (the same mechanism `postfix logrotate`
+# uses): postlogd keeps writing to the renamed inode until reload, and the
+# panel's log-tailer holds its own descriptor on that inode, so nothing
+# written before the reload is lost. `create` (rather than `nocreate`) matters
+# here beyond timing: a reload-triggered recreate lands the file at 0600,
+# which the unprivileged panel process cannot read — confirmed on a live
+# container — so logrotate must be the one to create it at 0644.
 #
 # logrotate itself only rotates once the configured "daily" period has elapsed
 # (tracked in /var/lib/logrotate/status), so it is safe to invoke this more

build/logrotate-mail.conf

@@ -5,5 +5,8 @@
 	notifempty
 	compress
 	delaycompress
-	copytruncate
+	create 0644 root root
+	postrotate
+		/usr/sbin/postfix reload
+	endscript
 }