Group cart tickets by event with +/- quantity buttons

- Cart page groups tickets by entry+type instead of listing individually
- Each group shows event name, type, date, qty with +/- buttons, line total
- New POST /cart/ticket-quantity/ route for adjusting from cart page
- Summary includes ticket quantities in Items count

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

app.py

@@ -28,6 +28,7 @@ from bp.cart.services.page_cart import (
     get_calendar_entries_for_page,
     get_tickets_for_page,
 )
+from bp.cart.services.ticket_groups import group_tickets
 
 
 async def _load_cart():
@@ -78,6 +79,7 @@ async def cart_context() -> dict:
         ctx["calendar_cart_entries"] = all_cal
         ctx["ticket_cart_entries"] = all_tickets
 
+    ctx["ticket_groups"] = group_tickets(ctx.get("ticket_cart_entries", []))
     ctx["total"] = total
     ctx["calendar_total"] = calendar_total
     ctx["ticket_total"] = ticket_total