fix(teams): wrap updateSplits delete and save in database transaction (#375)

[PINYOPATTANAWASANPORN]

Summary of Changes

• Wraps the member split replacement in TeamsService.updateSplits (src/teams/teams.service.ts) inside a database transaction via this.dataSource.transaction.
• Ensures that if inserting the new splits fails for any reason (constraint failure, connection drop, timeout), the deletion of the existing splits is rolled back, preventing the team from being left in a corrupted zero-split state.
• Adds comprehensive unit test coverage in src/teams/teams.service.spec.ts validating transactional isolation, validation ordering, and entity creation.

Root Cause / Technical Context

• Previously, TeamsService.updateSplits executed this.splitRepo.delete(...) followed by this.splitRepo.save(...) as separate, independent database calls outside of a shared transaction.
• While save() executes in an implicit transaction across its own inserted batch, a failure during save() left the earlier delete() already committed, stranding the team with 0 member splits.
• Fixes TeamsService.updateSplits' delete-then-insert isn't transactional — a mid-operation failure leaves a team with zero splits, silently killing its next payout #375.

Verification & Testing

• Surgical 2-file diff against upstream main (a89df16).
• Preserved strict Unix LF line endings.
• Added 3 unit tests in teams.service.spec.ts:
  1. Percentage validation failure rejects before entering the transaction.
  2. Successful replacement calls delete and save inside dataSource.transaction.
  3. Missing team throws NotFoundException before entering the transaction.

Impact & Compatibility

• Breaking changes: None.
• Dependencies affected: Injects DataSource into TeamsService using existing NestJS/TypeORM container bindings.

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