Persist per-season scoring rules

[adrunkhuman]

Summary

• Add per-season scoring-rule columns with default 3/1/0/0 values and startup migration for existing seasons.
• Make score calculation and every recalculation path use the fixture season's stored rules.
• Add guarded rule updates that preserve omitted values and block changes once a season has scores.
• Update user-facing scoring/help text and docs to avoid hardcoded fixed-rule claims.

Verification

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

Closes #50