Add ticket-to-cart integration

Reserved tickets now flow through the cart and checkout pipeline:
- TicketDTO gains price, entry_id, order_id, calendar_container_id
- CartSummaryDTO gains ticket_count, ticket_total
- 6 new CalendarService methods for ticket lifecycle
- cart_summary includes tickets; login adoption migrates tickets
- New _ticket_items.html template for checkout return page

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

contracts/dtos.py

@@ -55,6 +55,10 @@ class TicketDTO:
     calendar_name: str | None = None
     created_at: datetime | None = None
     checked_in_at: datetime | None = None
+    entry_id: int | None = None
+    price: Decimal | None = None
+    order_id: int | None = None
+    calendar_container_id: int | None = None
 
 
 @dataclass(frozen=True, slots=True)
@@ -127,3 +131,5 @@ class CartSummaryDTO:
     calendar_count: int = 0
     calendar_total: Decimal = Decimal("0")
     items: list[CartItemDTO] = field(default_factory=list)
+    ticket_count: int = 0
+    ticket_total: Decimal = Decimal("0")