Club come entità principale, branding e fix infrastruttura streaming.

Introduce clubs con abbonamento, branding Active Storage e flusso registrazione società; corregge healthcheck MediaMTX (immagine distroless) e allinea dominio produzione matchlivetv.it.

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

backend/app/models/club_membership.rb

@@ -0,0 +1,9 @@
+class ClubMembership < ApplicationRecord
+  ROLES = %w[owner].freeze
+
+  belongs_to :user
+  belongs_to :club
+
+  validates :role, inclusion: { in: ROLES }
+  validates :user_id, uniqueness: { scope: :club_id }
+end