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/migrations/init.ts

@@ -7,10 +7,19 @@ db.exec(`
     createdAt DATETIME NOT NULL,
     did TEXT NOT NULL,
     rkey TEXT NOT NULL,
+    listed INTEGER NOT NULL DEFAULT 1,
     PRIMARY KEY (did, rkey)
   );
 `);
 
+try {
+  db.exec(
+    `ALTER TABLE note ADD COLUMN listed INTEGER NOT NULL DEFAULT 1;`,
+  );
+} catch {
+  // Column already exists — no-op
+}
+
 db.exec(`
   CREATE TABLE IF NOT EXISTS state (
     key TEXT PRIMARY KEY,