fix(db): enable WAL mode, busy_timeout and single conn to avoid SQLITE_BUSY

internal/db/db.go

@@ -25,10 +25,19 @@ func Open(path string) (*DB, error) {
 	if err != nil {
 		return nil, fmt.Errorf("open sqlite: %w", err)
 	}
+	if _, err := conn.Exec(`PRAGMA journal_mode = WAL`); err != nil {
+		_ = conn.Close()
+		return nil, fmt.Errorf("set journal_mode: %w", err)
+	}
+	if _, err := conn.Exec(`PRAGMA busy_timeout = 5000`); err != nil {
+		_ = conn.Close()
+		return nil, fmt.Errorf("set busy_timeout: %w", err)
+	}
 	if _, err := conn.Exec(`PRAGMA foreign_keys = ON`); err != nil {
 		_ = conn.Close()
 		return nil, fmt.Errorf("enable foreign keys: %w", err)
 	}
+	conn.SetMaxOpenConns(1)
 	return &DB{conn: conn}, nil
 }