Breaking: Repository.aggregate() yields WrappedAggregate instead of Aggregate directly

What's Changed

• Event Sourcing: WrappedAggregate by @lzukowski in #108

Added

• In transaction subscription to category
• Custom context support in Repository.aggregate() via optional context parameter
• WrappedEvent.get_context() method for typed context retrieval

Fixes

• event_sourcery_sqlalchemy.configure_models annotation for sqlalchemy.orm.DeclarativeBase