refactor!: remove modelName option, resolve delegate key once in constructor

Replace the brittle uncapitalize(modelName) as "queueJob" pattern
(repeated 3 times) with a single validated delegate key stored in the
constructor. Add getModel(client) helper for transaction-scoped access.
The modelName option is removed since the library is typed against
Prisma.QueueJobDelegate and custom model names never truly worked.