Replace ticket qty input with +/- buttons, show sold/basket counts

- Entry page shows tickets sold count, remaining, and "in basket" count
- Replace numeric input + Buy button with add-to-basket / +/- controls
- New POST /tickets/adjust/ route creates/cancels tickets to target count
- Keep buy form active after adding (no confirmation replacement)
- New service functions: get_sold_ticket_count, get_user_reserved_count,
  cancel_latest_reserved_ticket

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

bp/tickets/services/tickets.py

@@ -182,6 +182,80 @@ async def get_tickets_for_entry(
     return result.scalars().all()
 
 
+async def get_sold_ticket_count(
+    session: AsyncSession,
+    entry_id: int,
+) -> int:
+    """Count all non-cancelled tickets for an entry (total sold/reserved)."""
+    result = await session.scalar(
+        select(func.count(Ticket.id)).where(
+            Ticket.entry_id == entry_id,
+            Ticket.state != "cancelled",
+        )
+    )
+    return result or 0
+
+
+async def get_user_reserved_count(
+    session: AsyncSession,
+    entry_id: int,
+    user_id: Optional[int] = None,
+    session_id: Optional[str] = None,
+    ticket_type_id: Optional[int] = None,
+) -> int:
+    """Count reserved tickets for a specific user/session + entry + optional type."""
+    filters = [
+        Ticket.entry_id == entry_id,
+        Ticket.state == "reserved",
+    ]
+    if user_id is not None:
+        filters.append(Ticket.user_id == user_id)
+    elif session_id is not None:
+        filters.append(Ticket.session_id == session_id)
+    else:
+        return 0
+    if ticket_type_id is not None:
+        filters.append(Ticket.ticket_type_id == ticket_type_id)
+    result = await session.scalar(
+        select(func.count(Ticket.id)).where(*filters)
+    )
+    return result or 0
+
+
+async def cancel_latest_reserved_ticket(
+    session: AsyncSession,
+    entry_id: int,
+    user_id: Optional[int] = None,
+    session_id: Optional[str] = None,
+    ticket_type_id: Optional[int] = None,
+) -> bool:
+    """Cancel the most recently created reserved ticket. Returns True if one was cancelled."""
+    filters = [
+        Ticket.entry_id == entry_id,
+        Ticket.state == "reserved",
+    ]
+    if user_id is not None:
+        filters.append(Ticket.user_id == user_id)
+    elif session_id is not None:
+        filters.append(Ticket.session_id == session_id)
+    else:
+        return False
+    if ticket_type_id is not None:
+        filters.append(Ticket.ticket_type_id == ticket_type_id)
+
+    ticket = await session.scalar(
+        select(Ticket)
+        .where(*filters)
+        .order_by(Ticket.created_at.desc())
+        .limit(1)
+    )
+    if ticket:
+        ticket.state = "cancelled"
+        await session.flush()
+        return True
+    return False
+
+
 async def get_available_ticket_count(
     session: AsyncSession,
     entry_id: int,