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_body.html

@@ -0,0 +1,7 @@
+{{define "logtail_body"}}
+<div id="logtail-body" hx-get="/logtail/body" hx-trigger="every 5s" hx-swap="outerHTML">
+  {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+  <span class="code">{{if .Lines}}{{range .Lines}}{{.}}
+{{end}}{{else}}No log lines yet.{{end}}</span>
+</div>
+{{end}}