Validate destination heat existence before advancing riders

[Copilot]

Addresses code review feedback from #62 requesting explicit validation of destination heats before advancing riders. Previously, addRiderToHeat silently returned when a destination heat didn't exist, masking potential data corruption bugs.

Changes

• heat-service.ts: Extract validateAndAddRiderToHeat() private method that checks destination heat exists via getHeatByHeatId() before calling addRiderToHeat(). Returns err(HeatDoesNotExistError) on missing heat.

• bracket-service.ts: Modify completeByeHeat() to return Result<void, HeatDoesNotExistError>, add destination heat validation, propagate errors to caller. Add HeatDoesNotExistError to BracketServiceError union.

Example

// Before: Silent failure if destination missing
if (metadata?.winnerDestinationHeatId) {
  await this.heatRepository.addRiderToHeat(metadata.winnerDestinationHeatId, winner.riderId);
}

// After: Explicit validation with typed error
if (metadata?.winnerDestinationHeatId) {
  const result = await this.validateAndAddRiderToHeat(
    metadata.winnerDestinationHeatId,
    winner.riderId
  );
  if (result.isErr()) {
    return err(result.error);
  }
}

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