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/models.py

@@ -185,6 +185,10 @@ class TableSeat(models.Model):
         "TarotCard", null=True, blank=True,
         on_delete=models.SET_NULL, related_name="significator_seats",
     )
+    deck_variant = models.ForeignKey(
+        "DeckVariant", null=True, blank=True,
+        on_delete=models.SET_NULL, related_name="active_seats",
+    )
 
 
 class DeckVariant(models.Model):