refactor: rename listed → discoverable on note

Pure rename — no behavioral change. "discoverable" more clearly
communicates that the field controls whether a note can be found
by others in public listings.

src/migrations/init.ts

@@ -7,7 +7,7 @@ db.exec(`
     createdAt DATETIME NOT NULL,
     did TEXT NOT NULL,
     rkey TEXT NOT NULL,
-    listed INTEGER NOT NULL DEFAULT 1,
+    discoverable INTEGER NOT NULL DEFAULT 1,
     PRIMARY KEY (did, rkey)
   );
 `);
@@ -20,6 +20,12 @@ try {
   // Column already exists — no-op
 }
 
+try {
+  db.exec(`ALTER TABLE note RENAME COLUMN listed TO discoverable;`);
+} catch {
+  // Column already renamed or doesn't exist — no-op
+}
+
 db.exec(`
   CREATE TABLE IF NOT EXISTS state (
     key TEXT PRIMARY KEY,