deck contribution sprint 1: TableSeat.deck_variant FK + select_role wiring — TDD

- epic.TableSeat gains deck_variant FK → DeckVariant (nullable, SET_NULL)
- select_role view assigns request.user.equipped_deck to seat on role confirmation
- Migration 0006_add_deck_variant_to_tableseat
- ITs: test_select_role_assigns_equipped_deck_to_seat,
       test_select_role_no_deck_leaves_deck_variant_null

Code architected by Disco DeDisco <discodedisco@outlook.com>
Git commit message Co-Authored-By:
Claude Sonnet 4.6 <noreply@anthropic.com>

src/apps/epic/views.py

@@ -575,6 +575,7 @@ def select_role(request, room_id):
             if room.table_seats.filter(role=role).exists():
                 return HttpResponse(status=409)
             active_seat.role = role
+            active_seat.deck_variant = request.user.equipped_deck
             active_seat.save()
         record(room, GameEvent.ROLE_SELECTED, actor=request.user,
                role=role, slot_number=active_seat.slot_number,