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/postfix/queue.go

@@ -0,0 +1,22 @@
+package postfix
+
+import (
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// Queue returns Postfix's own human-readable mail-queue listing (spec 7.2.11):
+// active, deferred and held messages, exactly as an administrator would see
+// via the CLI. The command takes a single fixed flag and no user input, so it
+// never goes through a shell (spec 7.6.3). The panel is responsible for
+// escaping the output before display (spec 7.6.7); this function returns it
+// as-is.
+func Queue() (string, error) {
+	cmd := exec.Command("postqueue", "-p")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return "", fmt.Errorf("postqueue -p: %w: %s", err, strings.TrimSpace(string(out)))
+	}
+	return string(out), nil
+}