Merge pull request #90 from PrivateStorageio/enforce-model-constraints

src/_zkapauthorizer/_storage_client.py

src/_zkapauthorizer/controller.py

src/_zkapauthorizer/model.py

src/_zkapauthorizer/tests/strategies.py

src/_zkapauthorizer/tests/test_controller.py

src/_zkapauthorizer/tests/test_storage_protocol.py

src/_zkapauthorizer/tests/test_storage_server.py