get_users() and get_release_logs() support

[maciekbanas]

Part of #754.

Implement remaining data retrieval methods for BitBucket Cloud:

• Users (get_users()): Retrieve user profiles via GET /2.0/users/{username}. Map to the standard gitstats_users table.
• Release logs (get_release_logs()): BitBucket doesn't have a native "releases" concept like GitHub/GitLab. Evaluate whether tags (GET /2.0/repositories/{workspace}/{repo_slug}/refs/tags) can serve as a proxy, or document this as unsupported.
• Unit tests with mocked API responses.