feat: sous-pages — parent_id, sidebar hiérarchique, bouton +New sub-page

- DB: parent_id dans table pages (ALTER TABLE migration auto)
- Sidebar: pages parent_id IS NULL affichées, sous-pages indentées en dessous
- API: create_page accepte parent_id (0 = racine)
- JS: newSubPage extrait page ID et passe parent_id
- page_editor: liste les sous-pages + bouton '+ New sub-page'

app/templates/page_editor.html

@@ -69,11 +69,36 @@
       <button class="sidebar-new-chat" style="width:auto; padding:8px 16px;" @click="insertMd('# Meeting Notes\n\n**Date:** \n**Attendees:** \n\n## Agenda\n\n- \n\n## Notes\n\n- \n\n## Action Items\n\n- [ ] \n', '')">🎤 Meeting Notes</button>
     </div>
   </div>
+
+  <!-- Sub-pages -->
+  <div style="margin-top:32px; border-top:1px solid var(--border); padding-top:16px;">
+    <h3 style="font-size:14px; font-weight:600; margin-bottom:12px;">Sub-pages</h3>
+    {% for sub in sub_pages %}
+    <a href="/pages/{{ sub.id }}" style="display:flex; align-items:center; gap:8px; padding:6px 8px; border-radius:6px; text-decoration:none; color:var(--text-primary); font-size:13px;"
+       onmouseover="this.style.background='var(--bg-hover)'" onmouseout="this.style.background='transparent'">
+      <span>📄</span> {{ sub.title }}
+    </a>
+    {% endfor %}
+    <button class="kanban-new-card" style="margin-top:8px;"
+            @click="createSubPage()">+ New sub-page</button>
+  </div>
 </div>
 {% endblock %}
 
 {% block scripts %}
 <script>
+  function createSubPage() {
+    const csrf = document.cookie.match(/csrf_token=([^;]+)/);
+    const token = csrf ? csrf[1] : '';
+    const project = document.querySelector('[x-data]')?.__x?.$data?.workspaceKey || '';
+    fetch(`/board/api/pages?title=New%20sub-page&section=Private&project=${encodeURIComponent(project)}&parent_id={{ page.id }}`, {
+      method: 'POST',
+      headers: { 'X-CSRF-Token': token }
+    })
+      .then(r => r.json())
+      .then(data => { window.location.href = `/pages/${data.id}`; })
+      .catch(err => { alert('Failed: ' + err.message); });
+  }
   // Simple Markdown-to-HTML renderer
   function renderMarkdown(md) {
     let html = md