ADR-001: Use PostgreSQL instead of MongoDB

Status

Accepted

Context

We need a relational database to support complex joins and ACID transactions. We considered MongoDB for its flexible schema but it lacks transactional guarantees across documents.

Decision

We'll use PostgreSQL.

Consequences

• Gains: strong consistency, mature tooling, powerful query engine.
• Costs: more up-front schema design, learning curve for some devs.