feat: sidebar tree navigation — toggle chevrons + drag & drop

- Add sort_order column to pages table for drag & drop reordering
- Rewrite _sidebar_data with recursive _build_page_tree for nested hierarchy
- Add tree toggle buttons (▶ collapsed / ▼ expanded) on parent items
- Alpine.js expandedNodes state with toggleTreeNode() for expand/collapse
- Jinja2 recursive macro render_tree_item for proper nesting + indentation
- SortableJS integration on .sidebar-items for drag & drop repositioning
- New PUT /board/api/pages/{id}/move API for persisting moves
- Auto-set sort_order on page creation (max+1 for same parent)
- CSS: .tree-toggle, .tree-toggle-placeholder, grab cursor on page-icon
- 28/28 tests pass, all endpoints healthy

app/db.py

@@ -147,6 +147,11 @@ def init_db():
             conn.execute("ALTER TABLE pages ADD COLUMN parent_id INTEGER REFERENCES pages(id)")
         except sqlite3.OperationalError:
             pass
+        # Migration: add sort_order for drag & drop tree reordering (v1.1.0+)
+        try:
+            conn.execute("ALTER TABLE pages ADD COLUMN sort_order INTEGER NOT NULL DEFAULT 0")
+        except sqlite3.OperationalError:
+            pass
         conn.commit()