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>

internal/web/templates/dashboard.html

@@ -3,6 +3,9 @@
   <h1>SelfPost</h1>
   <div class="actions muted">
     <span>{{.User}}</span>
+    <a href="/sendlog">Send log</a>
+    <a href="/queue">Queue</a>
+    <a href="/logtail">Log</a>
     <form class="inline" method="post" action="/reload">
       <button type="submit">Reload</button>
     </form>