Schema Diff

Diff old vs new schema. Flag by risk:

• Destructive (high): dropped column/table, narrowed type, new NOT NULL on existing rows, dropped index a query relies on.
• Locking (high at scale): index without CONCURRENTLY, type change that rewrites the table.
• Safe: new nullable column, new table, new index CONCURRENTLY. For each destructive change: is data lost? is it reversible? is there a backfill? Output a risk-ranked list and a go/no-go with the safe rollout order.