fix: SQLite busy_timeout prevents 'database is locked' during parallel build

Next.js uses multiple workers for prerendering, all hitting the same
SQLite DB file. WAL mode allows concurrent reads but writes still lock.
Added busy_timeout=5000 so workers wait up to 5s instead of failing
immediately with 'database is locked'.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 7418

src/lib/db.ts

@@ -51,6 +51,7 @@ export function getDb(): Database.Database {
 
     db = new Database(DB_PATH);
     db.pragma('journal_mode = WAL');
+    db.pragma('busy_timeout = 5000');
     db.pragma('foreign_keys = ON');
     initDb(db);
   }