Add test infrastructure for hydrators using node:test

- Add docker-compose.test.yml with isolated PostgreSQL on port 5433
- Add environment variable support for database connection config
- Add test setup utilities and initial user hydrator tests
- Add test and test:watch scripts to package.json

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.test.yml

@@ -0,0 +1,13 @@
+name: diachron-test
+
+services:
+  postgres:
+    image: postgres:17
+    ports:
+      - "5433:5432"
+    environment:
+      POSTGRES_USER: diachron_test
+      POSTGRES_PASSWORD: diachron_test
+      POSTGRES_DB: diachron_test
+    tmpfs:
+      - /var/lib/postgresql/data