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/base.html

@@ -436,7 +436,8 @@
       },
       newSubPage(parentId) {
         const project = this.workspaceKey || '';
-        fetch(`/board/api/pages?title=New%20page&section=Private&project=${encodeURIComponent(project)}`, {
+        const pid = parentId.replace('page/', '');
+        fetch(`/board/api/pages?title=New%20sub-page&section=Private&project=${encodeURIComponent(project)}&parent_id=${pid}`, {
           method: 'POST',
           headers: { 'X-CSRF-Token': this.getCsrfToken() }
         })