fix: add busy_timeout pragma to prevent "database is locked" errors

SQLite was immediately returning SQLITE_BUSY when concurrent writes
collided (e.g., recordActivity firing from a timer during another write).
Adding a 5-second busy_timeout lets SQLite retry internally before failing.

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

packages/core/src/db.ts

@@ -439,6 +439,8 @@ export class Database {
 
     // Enable WAL mode for concurrent reader/writer access
     this.db.exec("PRAGMA journal_mode = WAL");
+    // Wait up to 5s for locks to clear before returning SQLITE_BUSY
+    this.db.exec("PRAGMA busy_timeout = 5000");
     // Enable foreign key enforcement
     this.db.exec("PRAGMA foreign_keys = ON");
   }