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>

internal/web/handlers_monitor.go

@@ -1,6 +1,8 @@
 package web
 
 import (
+	"errors"
+	"io/fs"
 	"net/http"
 	"strconv"
 
@@ -164,6 +166,12 @@ func (s *Server) handleLogTailBody(w http.ResponseWriter, r *http.Request) {
 func (s *Server) readLogTail() ([]string, string) {
 	lines, err := logtail.TailLines(s.cfg.MailLogPath, logTailLines)
 	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			// Rotation renamed the file away; Postfix recreates it on reload
+			// (within about a second), so this is a normal, brief gap rather
+			// than a failure worth alarming the operator about.
+			return nil, ""
+		}
 		logf("panel: tail %s: %v", s.cfg.MailLogPath, err)
 		return nil, "Could not read the mail log."
 	}