No persisted tournament champion / completed state #103

Problem / motivation

ValidateNotFinalRound detects that the final has been played (winners.Count < 2) and blocks further UpdateRound calls, but this is purely a guard — nothing ever writes a "champion" anywhere. Neither Bracket nor Tournament has a WinnerId/ChampionId field, BracketType exposes only Id, TournamentId, Matches, and TournamentStatus never transitions to any Completed/Finished value (it's only Open/Closed). A client has to independently find the highest-round match and read its WinnerId to determine the tournament winner.

What should happen

• Add a WinnerId field to Tournament, populated once the final match is decided.
• Add a Completed/Finished value to TournamentStatus and transition to it at that point.
• Expose the winner through TournamentType so clients don't need to derive it themselves.

Affected files

Brackets/BracketMutationValidations.cs:28-29 (ValidateNotFinalRound), Data/Models/Tournament.cs, Data/Models/TournamentStatus.cs, Tournaments/TournamentType.cs