Phase 7: monitoring UI — send log, queue, mail.log tail

Three HTMX-polled monitoring screens (spec 7.2.11-13): send log with
server-side domain/application filters and pagination, Postfix queue
(postqueue -p), and a mail.log tail. Fragment endpoints return HTML
snippets, not JSON (spec 7.1); all output is auto-escaped via
html/template (spec 7.6.7).

Adds store.QuerySendLog/CountSendLog/ListApplicationLogins,
postfix.Queue(), and logtail.TailLines (a point-in-time reverse read,
independent of the background follow loop). Verified on the dev server:
gofmt/vet/test green, docker build green, container e2e (filters,
60-row pagination, <script> escaping, real postqueue/mail.log output,
existing Reload button unaffected).

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

cmd/panel/httpserver.go

@@ -27,6 +27,7 @@ func serveHTTP(ctx context.Context, cfg config, st *store.Store) error {
 	srvApp, err := web.New(st, domains, apps, web.Config{
 		Hostname:     cfg.hostname,
 		CookieSecure: cfg.cookieSecure,
+		MailLogPath:  cfg.mailLog,
 	}, cfg.setupTokenPath)
 	if err != nil {
 		return err