fix: preserve pieceId during deal creation

[silent-cipher]

Summary

These changes address pieceId preservation issues. uploadResult is often missing pieceId, so we preserve it from the piecesConfirmed event. This is important for cleaning up pieces later on.

Problem

Missing pieceId while testing locally -

Select count(*) from deals where status = 'deal_created' and piece_id is null;
 count 
-------
    25
(1 row)

Select count(*) from deals where status = 'deal_created';
 count 
-------
    68
(1 row)

[Copilot]

Pull request overview

This PR updates the backend deal-creation flow to better preserve pieceId when executeUpload() returns an uploadResult without it, using the onPiecesConfirmed progress event as a fallback so later piece cleanup remains possible.

Changes:

• Capture pieceId from the onPiecesConfirmed upload progress event (fallback source).
• Avoid overwriting an already-set deal.pieceId with a missing uploadResult.pieceId.
• Update unit test typings to model pieceIds as bigint[] and emit a bigint in the mocked progress event.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
apps/backend/src/deal/deal.service.ts	Sets deal.pieceId during onPiecesConfirmed and preserves it when uploadResult.pieceId is nullish.
apps/backend/src/deal/deal.service.spec.ts	Adjusts mocked upload progress event typing/data to use bigint[] for pieceIds.

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