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/db.py

@@ -137,10 +137,16 @@ def init_db():
                 title TEXT NOT NULL DEFAULT 'New page',
                 content TEXT NOT NULL DEFAULT '',
                 parent_section TEXT DEFAULT 'Private',
+                parent_id INTEGER REFERENCES pages(id),
                 created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
                 updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
             );
         """)
+        # Migration: add parent_id if missing (v1.0.0+)
+        try:
+            conn.execute("ALTER TABLE pages ADD COLUMN parent_id INTEGER REFERENCES pages(id)")
+        except sqlite3.OperationalError:
+            pass
         conn.commit()