Previously, each chunk and blob_chunk was inserted in a separate
transaction, leading to ~560k+ transactions for large backups.
This change batches all database operations per blob:
- Chunks are queued in packer.pendingChunks during file processing
- When blob finalizes, one transaction inserts all chunks, blob_chunks,
and updates the blob record
- Scanner tracks pending chunk hashes to know which files can be flushed
- Files are flushed when all their chunks are committed to DB
- Database is consistent after each blob finalize
This reduces transaction count from O(chunks) to O(blobs), which for a
614k file / 44GB backup means ~50-100 transactions instead of ~560k.
sneak
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