Add admin season management

[adrunkhuman]

Summary

• Show the active season in the unified admin panel and add a New Season modal action.
• Add atomic season rollover that archives the previous active season and blocks unsafe transitions while fixtures are open.
• Cover lifecycle, rollback, panel refresh, and week-reset behavior with tests.

Verification

• uv run pytest --tb=short
• uv run ruff check .
• uv run ruff format --check .
• uv run ty check typer_bot

Closes #219