Clickhouse Migrations

This codifies the migration patterns for ClickHouse databases, specifically the rules around sharded versus replicated tables, node roles, and table engines. You'll want this when you're working on a PostHog-style setup or any complex ClickHouse deployment where you need to coordinate migrations across clusters. The key thing it enforces is avoiding the ON CLUSTER clause and always using IF EXISTS guards, which prevents the kind of race conditions that'll wreck distributed database migrations. It's opinionated about structure, which is good because ClickHouse migration footguns are real. The cheat sheet format makes it fast to reference whether you need NodeRole.DATA or NodeRole.INGESTION_SMALL without digging through docs.