fix(logtail): keep mail.log in /data and reconcile stuck rows (v1.x closure phase 2)

Move the delivery log from the ephemeral /var/log to /data/log/mail.log so
the lines that resolve a queued send-log row survive a container recreate.
postlogd writes it as postfix, the panel reads it through the selfpost group
(dir 2750, file 0640, normalised every start); backups exclude log/.

Close the residual gap with a queue sweep: rows queued for over two minutes
whose id postqueue -p no longer lists are marked bounced. The sweep waits
until the tailer has read the log to its end and does nothing when the queue
cannot be listed, so a message in flight is never touched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/panel/envdoc_test.go

@@ -82,6 +82,7 @@ var buildScriptKeys = []string{
 	"RATE_LIMIT_WINDOW_SECONDS",
 	"OPENDKIM_SOCKET",
 	"JOURNAL_MILTER_SOCKET",
+	"MAIL_LOG",
 	"POSTFIX_SENDER_LOGIN_MAPS",
 	"SASL_DB_PATH",
 	"SUBMISSION_ENABLE",