openai/gpt-chat-latest → A (5:1)

Side A extracts duplicated test infrastructure into shared utilities (`test.common` and `test.oauth`), introducing reusable helpers such as `run-cargo-build-release!`, `slug-server-env`, `complete-registration!`, shared assertions, configurable mock OAuth users, and consistent HTTP timeouts while updating multiple test suites to use them. Side B mainly improves the local development fixture workflow (cargo-watch, persistent fixture data, preferred port) and removes a small piece of room-page UI, which is useful but has a narrower, less foundational impact on the project's long-term maintainability.