Use PostgreSQL as the primary datastore

Context

We need a primary datastore for the new billing service. The team has production experience with both PostgreSQL and MongoDB, and the data model is relational (accounts, invoices, line items with foreign keys).

Decision

We will use PostgreSQL 16, managed via RDS, as the primary datastore for all new services unless a specific workload has a strong document-store or time-series justification.

Consequences

• Schema migrations require a formal review (see docs/migrations).
• We standardize on one connection-pooling strategy (PgBouncer) across services, simplifying on-call runbooks.
• Document-shaped data (e.g. audit logs) will need a deliberate exception request rather than defaulting to Postgres JSONB columns.