Aggiunge i tornei con hub, pagina pubblica e tabellone per semifinali e finali.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/models/tournament_group.rb

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+class TournamentGroup < ApplicationRecord
+  belongs_to :tournament
+  has_many :participants, class_name: "TournamentParticipant", foreign_key: :group_id, dependent: :nullify
+  has_many :matches, dependent: :nullify
+
+  validates :name, presence: true
+  validates :position, numericality: { only_integer: true, greater_than_or_equal_to: 0 }
+end