Add Reservation.IsActive() and SKU.IsInStock()

Callers who want to know "is this reservation still holding stock?" have to compare r.Status == inventory.ReservationActive themselves; callers who want "does this SKU have anything left to sell?" have to compare sku.Available > 0 themselves. Both are one-line checks, but they are duplicated at every call site instead of being named, self-documenting methods on the domain types themselves -- every future caller (HTTP handlers, the load-test tool, a future admin CLI) will otherwise reimplement them slightly differently each time.

There is currently no Reservation.IsActive() or SKU.IsInStock() method; the panicking stub TODO(ticket-07) is called by tests wherever either is needed.

1. go test ./tickets/tests/... -run TestTicket07 -v
2. Observe: both subtests panic with "TODO(ticket-07): Reservation.IsActive not implemented" and "TODO(ticket-07): SKU.IsInStock not implemented".

This is a small ergonomic gap in the domain types (pkg/inventory) that every future caller -- HTTP handlers, the load-test tool, a future admin CLI -- will otherwise reimplement slightly differently each time. Centralizing the definition here means "active" and "in stock" mean exactly one thing across the whole codebase.

Both are one-line comparisons against fields already on the struct -- no database access, no new imports. Look at how ReservationStatus and its ReservationActive constant are already defined right above Reservation in this same file.