feat: add listed field to note for public listing visibility

Notes with listed=false are filtered out from all GET /notes queries.
The field defaults to true so existing and new notes without it remain visible.
Migration handles existing databases with ALTER TABLE ADD COLUMN.

src/data/note.ts

@@ -4,4 +4,5 @@ export type Note = {
   title: string;
   publishedAt: string;
   createdAt: string;
+  listed?: boolean;
 };