v0.2.0 Multi-league foundation

Highlights

• Added the League model and default-league migration path for existing data.
• Added league-aware team lists, leaderboards, QR set management, QR generation, and redemption checks.
• Added a senior-developer inspection prompt and roadmap for the next milestones.
• Replaced brittle admin-dashboard mocks with in-memory behavior tests.
• Removed the unused mysqlclient dependency; the app uses pymysql.

Validation

• GitHub Docker workflow passed on PR #143.
• GitGuardian and CodeRabbit checks passed on PR #143.
• Local: python3 -m compileall app tests.
• Local: .venv/bin/python -m pip install -r requirements.txt.
• Local: .venv/bin/python -m pytest -q.