Design persisted-data models that enforce the domain's real invariants — relational schemas, document stores, key-value and wide-column databases, and property graphs. Covers keys, relationship direction, partitioning, normalization versus duplication, constraints, traversal and access patterns, and additive/backward-compatible migrations. Triggers when designing or changing stored data in any database family. Narrower than `backend-engineering` (trusted-side runtime laws — secrets, validation, auth) — this is specifically about the shape of persisted data, not the code that reads and writes it.
