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/logtail.html

@@ -0,0 +1,21 @@
+{{define "content"}}
+<div class="topbar">
+  <h1>Mail log</h1>
+  <div class="actions muted">
+    <span>{{.User}}</span>
+    <a href="/">Domains</a>
+    <a href="/sendlog">Send log</a>
+    <a href="/queue">Queue</a>
+    <form class="inline" method="post" action="/logout">
+      <button type="submit">Sign out</button>
+    </form>
+  </div>
+</div>
+
+<a class="back" href="/">&larr; Domains</a>
+
+<div class="card">
+  <h2>mail.log tail</h2>
+  {{template "logtail_body" .}}
+</div>
+{{end}}