D3 — Database engine: Postgres everywhere, including tests
Architecture decision record, relocated verbatim from the retired single-file
docs/tenancy/decision log. Status, thematic clusters, and how to record a new ADR: the decision log index.
config/database.php previously defaulted to sqlite; .env.example already sets
DB_CONNECTION=pgsql. RLS is Postgres-only, so tenant-isolation tests must run on
Postgres. We will run the test suite against a dedicated Postgres test database for
prod parity (see D8). The sqlite default in config/database.php will be changed to
pgsql.