get_issues() support

[maciekbanas]

Part of #754.

Implement get_issues() for BitBucket Cloud:

• BitBucket issue tracker is optional per-repo (must be enabled in repo settings). Handle repos that have it disabled gracefully.
• Retrieve issues via GET /2.0/repositories/{workspace}/{repo_slug}/issues.
• Map to the standard gitstats_issues table: number, title, description, created_at, closed_at, state, url, author, repo_name, organization, api_url.
• Unit tests with mocked API responses.