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>

services/stubs.py

@@ -98,6 +98,37 @@ class StubCalendarService:
     ) -> dict[int, list[CalendarEntryDTO]]:
         return {}
 
+    async def pending_tickets(
+        self, session: AsyncSession, *, user_id: int | None, session_id: str | None,
+    ) -> list[TicketDTO]:
+        return []
+
+    async def tickets_for_page(
+        self, session: AsyncSession, page_id: int, *, user_id: int | None, session_id: str | None,
+    ) -> list[TicketDTO]:
+        return []
+
+    async def claim_tickets_for_order(
+        self, session: AsyncSession, order_id: int, user_id: int | None,
+        session_id: str | None, page_post_id: int | None,
+    ) -> None:
+        pass
+
+    async def confirm_tickets_for_order(
+        self, session: AsyncSession, order_id: int,
+    ) -> None:
+        pass
+
+    async def get_tickets_for_order(
+        self, session: AsyncSession, order_id: int,
+    ) -> list[TicketDTO]:
+        return []
+
+    async def adopt_tickets_for_user(
+        self, session: AsyncSession, user_id: int, session_id: str,
+    ) -> None:
+        pass
+
 
 class StubMarketService:
     async def marketplaces_for_container(