Refactor fixtures and library code for requests

[jbrunton]

• Reusable fixtures for repos and request/response processing.
• Maybe some shared types or library code for request processing, to (a) provide a common structure (parse request -> process request -> generate response) and (b) DRY up code if possible.
• Need some middleware for handling exceptions – which might include a switch to fp-ts RTE monad.

[jbrunton]

See:

• #147 for reusable fixtures
• #152 for refactored pipeline code
• #155 for exception handling