Test quality improvements

[streaky]

This pull request introduces a comprehensive integration test harness for the ACME API, enabling end-to-end testing against a real Pebble ACME server. It also includes several improvements to configuration, documentation, and testing practices.

Integration test harness and related infrastructure:

• Added a Docker Compose-based integration harness (docker-compose.harness.yaml) to spin up a Pebble ACME server, challenge server, and test instance of the API for end-to-end testing.
• Implemented a Python script (tests/integration/pebble_harness/run_harness.py) to orchestrate the harness lifecycle: setup, health check, test execution, and teardown.
• Added configuration files for the harness, including acme.api.test-config.yaml for the API and pebble-config.json for Pebble, to support isolated integration tests. [1] [2]

Testing and Makefile enhancements:

• Added a test-harness target to the Makefile to automate running the integration harness, and updated .PHONY targets accordingly. [1] [2]
• Moved and fixed the unit test Alembic migration test path for better test organization.

Configuration and documentation improvements:

• Changed check_interval_hours in RenewalConfig to a float and allowed values greater than zero for more flexible renewal scheduling.
• Updated documentation to clarify that linter warnings should not be silenced without strong justification and must be commented if so.
• Expanded the security warning in README.md to emphasize that the service is experimental and should only be used with strict access controls.