Fix TypeError: Decimal + float in cart_total computation

ticket_total() returned float while total() returns Decimal, causing
a crash when summing cart totals. Standardise all total functions to
return Decimal.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bp/cart/services/calendar_cart.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from decimal import Decimal
+
 from shared.services.registry import services
 from .identity import current_cart_identity
 
@@ -17,12 +19,12 @@ async def get_calendar_cart_entries(session):
     )
 
 
-def calendar_total(entries) -> float:
+def calendar_total(entries) -> Decimal:
     """
     Total cost of pending calendar entries.
     """
     return sum(
-        (e.cost or 0)
+        (Decimal(str(e.cost)) if e.cost else Decimal(0))
         for e in entries
         if e.cost is not None
     )
@@ -38,6 +40,6 @@ async def get_ticket_cart_entries(session):
     )
 
 
-def ticket_total(tickets) -> float:
+def ticket_total(tickets) -> Decimal:
     """Total cost of reserved tickets."""
-    return sum(float(t.price or 0) for t in tickets)
+    return sum((Decimal(str(t.price)) if t.price else Decimal(0) for t in tickets), Decimal(0))