db: drizzle schema matching spec 02, initial migrations, settings service

Migration 0000 enables citext; 0001 creates all 39 tables. DB tests use a
throwaway database per file (TEST_DATABASE_URL, derived from .env in dev,
a postgres service in CI). Decision 0002: pg driver and @fastify/cookie.

Task: M1-T1

docs/decisions/0002-pg-and-cookie.md

@@ -0,0 +1,5 @@
+# 0002 — `pg` driver and `@fastify/cookie`
+
+drizzle needs a PostgreSQL driver. We use `pg` (node-postgres): the most used Node driver, and the
+one drizzle's docs and migrator target first. `@fastify/cookie` reads and sets the `sp_session`
+cookie (04 §2.2); spec 04 §1 already names a cookie plugin but 00 §4 did not list it.