Refactor order-processor service to use FastAPI. Include unit tests

[hbuie112358]

Describe the refactor and reasoning

Refactor the service from Flask to FastAPI to take advantage of native async support. This work will serve as the initial test case for the upcoming CI pipeline.
Tasks

Refactor: Convert all Flask routes and Dapr client calls to FastAPI/Async.

Test: Write a pytest suite for the new asynchronous endpoints.

Verify: Ensure tests pass locally on the System76 Gazelle.

Success Criteria

[ ] Service is fully functional on FastAPI.

[ ] Unit tests are ready to be used as a "gate" for the CI pipeline (to be configured in a separate issue).