feat: add Docker Compose + TestContainers for PostgreSQL

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,14 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_DB: cameleer_saas
+      POSTGRES_USER: cameleer
+      POSTGRES_PASSWORD: cameleer_dev
+    ports:
+      - "5432:5432"
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+
+volumes:
+  pgdata: