Phase 2+5: lab submissions and student lab flow

- progress: submissions table+V4; SubmissionService (deadline-gated submit,
  unlimited resubmits, per-student history, zip download); files on disk
  data/submissions/{student}/{lab}/{seq}/
- student API: POST submit (multipart), GET me submissions, download; admin
  GET lab submissions + admin download (zip pack)
- progress enrich(): labId in MeAssignment; web DTO aligned
- web: /student dashboard (avg/best/worst cards), /student/labs cards,
  /student/labs/{id}: condition md, variant block, submit form, history;
  'Лабораторные' link in nav for students
- admin: submissions column on lab assignment table + download link
- fixed long-standing template bug: broken @{} expression (missing }) in
  admin/lab and student/lab

web/src/main/resources/templates/admin/lab.html

@@ -98,7 +98,7 @@
         <h3 class="group-title" th:text="${g.groupName}">Группа</h3>
         <div class="card card-flush mb-6">
             <table>
-                <thead><tr><th>Студент</th><th>Логин</th><th>Уровень</th><th>Вариант</th><th>Источник</th></tr></thead>
+                <thead><tr><th>Студент</th><th>Логин</th><th>Уровень</th><th>Вариант</th><th>Источник</th><th>Сдача</th></tr></thead>
                 <tbody>
                 <tr th:each="s : ${g.students}">
                     <td th:text="${s.fullName}">—</td>
@@ -130,8 +130,12 @@
                     </td>
                     <td><span th:if="${s.variantNumber != null}" class="font-mono" th:text="${s.variantNumber}">—</span></td>
                     <td><span class="badge" th:classappend="${s.source == 'вручную'} ? 'badge-warn' : 'badge-ok'" th:text="${s.source}">—</span></td>
+                    <td><a th:if="${s.submissionId}" class="badge badge-ok"
+                           th:href="'/admin/submissions/' + ${s.submissionId} + '/download'"
+                           th:text="|попытка ${s.submissionSeq}|">↓</a>
+                        <span th:unless="${s.submissionId}" class="text-muted">не сдано</span></td>
                 </tr>
-                <tr th:if="${#lists.isEmpty(g.students)}"><td colspan="6" class="empty">В группе нет студентов.</td></tr>
+                <tr th:if="${#lists.isEmpty(g.students)}"><td colspan="7" class="empty">В группе нет студентов.</td></tr>
                 </tbody>
             </table>
         </div>