fix: v24 migration error message points to actual snapshot path

The pre-migration snapshot was correctly migrated to STATE_DIR-aware
path in src/db.py:1832 (`_get_state_dir() / 'system.duckdb.pre-migrate'`),
but the error message in _migrate_v24_bq_source_queries still
hardcoded the old `{DATA_DIR}/state/...` shape. Under flat-mount
layout (STATE_DIR=/data-state), an operator hitting the v24
migration error would look in /data/state/ for a rollback snapshot
that lives in /data-state/. Devin Review on PR #194 round 3.

src/db.py

@@ -1771,7 +1771,7 @@ def _v23_to_v24_finalize(conn: duckdb.DuckDBPyConnection) -> None:
             f"`instance.yaml: data_source.bigquery.project`) and restart "
             f"the app to retry the migration. The schema version is NOT "
             f"bumped to 24 until this completes; pre-migration DB "
-            f"snapshot is at `{{DATA_DIR}}/state/system.duckdb.pre-migrate`."
+            f"snapshot is at `{_get_state_dir()}/system.duckdb.pre-migrate`."
         )
 
     conn.execute("BEGIN TRANSACTION")