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/routers/dashboard.py

@@ -71,7 +71,13 @@ async def view_page_root(request: Request, page_id: int):
     parts = ws.split("/") if "/" in ws else ["", ""]
     owner, repo = parts[0], parts[1] if len(parts) > 1 else ""
     sidebar = board_sidebar(request, owner, repo)
-    ctx = {**sidebar, "page": page}
+    # Load sub-pages
+    with get_conn() as conn:
+        subs = conn.execute(
+            "SELECT id, title FROM pages WHERE parent_id=? ORDER BY updated_at DESC",
+            (page_id,),
+        ).fetchall()
+    ctx = {**sidebar, "page": page, "sub_pages": [dict(s) for s in subs]}
     template = env.get_template("page_editor.html")
     return template.render(**ctx)