store: Add resumable entity data import for restore

Add import_data() to the restore module that reads Parquet chunks
and inserts entity data into PostgreSQL tables. Supports resumability
by checking max(vid) in each table and skipping already-imported rows.

Entity tables use InsertQuery::for_restore() for efficient batch
inserts. The data_sources$ table uses raw SQL with bind parameters
since it has a fixed schema outside the Layout.