setup_sig_session: wire deck contributions; _room_deck_variant replaces owner.equipped_deck

- setup_sig_session: drop _ensure_earthman() (deck seeded by migration); set
  deck_variant=earthman on all TableSeats; users get unlocked_decks add but
  equipped_deck=None (seat owns the deck); docstring documents role-pair mapping
- _room_deck_variant(room): new helper looks up deck from any seated deck_variant,
  falls back to owner.equipped_deck for legacy rooms
- sig_deck_cards / _sig_unique_cards: use _room_deck_variant instead of
  owner.equipped_deck — sig cards now work even when users have unequipped their deck
  after role confirmation

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

@@ -421,6 +421,19 @@ class SigReservation(models.Model):
 
 # ── Significator deck helpers ─────────────────────────────────────────────────
 
+def _room_deck_variant(room):
+    """Return the DeckVariant in use for this room.
+
+    Looks up the deck committed to any TableSeat in the room (all seats share the
+    same deck per game).  Falls back to the room owner's equipped_deck for rooms
+    created before deck contribution was wired.
+    """
+    seat = room.table_seats.filter(deck_variant__isnull=False).first()
+    if seat:
+        return seat.deck_variant
+    return room.owner.equipped_deck
+
+
 def sig_deck_cards(room):
     """Return 36 TarotCard objects forming the Significator deck (18 unique × 2).
 
@@ -429,7 +442,7 @@ def sig_deck_cards(room):
     NC/EC pair → MAJOR arcana numbers 0 and 1:                               2 unique
     Total: 18 unique × 2 (levity + gravity piles) = 36 cards.
     """
-    deck_variant = room.owner.equipped_deck
+    deck_variant = _room_deck_variant(room)
     if deck_variant is None:
         return []
     wands_crowns = list(TarotCard.objects.filter(
@@ -455,7 +468,7 @@ def sig_deck_cards(room):
 
 def _sig_unique_cards(room):
     """Return the 18 unique TarotCard objects that form one sig pile."""
-    deck_variant = room.owner.equipped_deck
+    deck_variant = _room_deck_variant(room)
     if deck_variant is None:
         return []
     wands_crowns = list(TarotCard.objects.filter(