feat: batched migration for blobs.media_type_id_convert_to_bigint backfill

Relates to #1707 (closed)

Changes

This MR implements the batched background migration for backfilling the blobs.media_type_id_convert_to_bigint column as part of Phase 1 of the Registry BBM Procedure for BIGINT media types column swap (Epic &19732).

Expected Behavior

When this migration runs:

1. 64 BBM records will be created in the batched_background_migrations table
2. The BBM worker will execute the copyBlobMediaTypeIDToNewBigIntColumn work function
3. Each partition will be processed independently with batches of 100,000 rows (each with 20k subbatch)
4. The media_type_id_convert_to_bigint column will be backfilled from media_type_id for all rows
5. The existing trigger ensures new/updated rows stay in sync automatically

This implementation is a prerequisite for Phase 2 (creating indexes and foreign keys) and Phase 3 (column swap).