chore: remove obsolete startup migrations

[adrunkhuman]

Summary

• Remove automatic startup migrations/backfills for historical database shapes
• Keep fresh database initialization, but require existing databases to already match the current schema
• Add startup validation for required columns, required conflict-target unique constraints, duplicate result rows, and invalid fixture guild ownership
• Replace legacy migration-contract tests with current-schema restart and fail-fast validation tests

Live DB Verification

Ran the schema inspection command from #232 against /app/data/typer.db before opening this PR.

Observed:

• required current columns exist for seasons, fixtures, predictions, results, scores, and guild_config
• required unique indexes exist for predictions(fixture_id, user_id), results(fixture_id), scores(fixture_id, user_id), and guild_config(guild_id)
• existing extra fixture columns announcement_message_id and thread_id are harmless and ignored
• prior data-state check confirmed one active season, no fixtures without season_id, no foreign season references, and valid guild_config.active_season_id

Verification

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

Closes #232