Backfill orchestrator should support batched execution #570

Context

The batch_size parameter was removed from the backfill orchestrator and backfill_radionuclides() in #558 because it was accepted but never used — all rows were loaded and committed in a single transaction.

Work Required

Implement real batched execution for backfill jobs:

• Process rows in configurable batch sizes using LIMIT/OFFSET or cursor-based pagination
• Commit after each batch so partial progress survives failures
• Re-add --batch-size CLI flag once batching is implemented
• Log per-batch progress

Refs

Removed in #558