Allow more input options when creating stage items

[evroon]

ref https://github.com/evroon/bracket/discussions/611

Right now, you can only select the winners up to the 4th position for a subsequent stage item. Number 5 and up cannot be selected. We should allow that.

- 1st Group A vs 2nd Group B
- 2nd Group A vs 1st Group B
- 3rd Group A vs 4th Group B
- 4th Group A vs 3rd Group B
- 5th Group A vs 6th Group B
- 6th Group A vs 5th Group B

The last two options here aren't available.

We can either add all possible options to the dropdown, or set the max as a config value instead of hardcoded, or allow the user to set the max in the frontend.

The limit of four is hardcoded here:

https://github.com/evroon/bracket/blob/b395d148f0f8899926a30f0f4bb178974403d720/backend/bracket/logic/scheduling/builder.py#L91-L96

[github-actions[bot]]

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