Feat: Enforce PostgreSQL for integration tests; add Docker test stack

- conftest.py: pytest_configure guard rejects non-postgresql+asyncpg:// URLs
  before any test collects (per constitution §2.5/§5.2 v1.3.0)
- docker-compose.test.yml: isolated postgres-test (5433) + minio-test (9002)
  + api-test runner; one command runs the full suite against real PostgreSQL
- Makefile: test-unit and test-integration targets
- .env.test.example: documents variables needed to run tests outside Docker
- Fix pre-existing test bug: integration tests using client fixture (NoOpAuthProvider)
  for write operations (upload/delete/patch) now use authed_client with Bearer
  token — these were never caught because tests never ran against a live stack

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.test.yml

@@ -0,0 +1,67 @@
+services:
+  postgres-test:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: reactbin
+      POSTGRES_PASSWORD: reactbin
+      POSTGRES_DB: reactbin_test
+    ports:
+      - "5433:5432"
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U reactbin"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+  minio-test:
+    image: minio/minio:latest
+    command: server /data --console-address ":9001"
+    environment:
+      MINIO_ROOT_USER: minioadmin
+      MINIO_ROOT_PASSWORD: minioadmin
+    ports:
+      - "9002:9000"
+      - "9003:9001"
+    healthcheck:
+      test: ["CMD", "mc", "ready", "local"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+  minio-init-test:
+    image: minio/mc:latest
+    depends_on:
+      minio-test:
+        condition: service_healthy
+    environment:
+      MINIO_ROOT_USER: minioadmin
+      MINIO_ROOT_PASSWORD: minioadmin
+    entrypoint: >
+      /bin/sh -c "
+      mc alias set local http://minio-test:9000 $$MINIO_ROOT_USER $$MINIO_ROOT_PASSWORD &&
+      mc mb --ignore-existing local/reactbin-test
+      "
+
+  api-test:
+    build:
+      context: ./api
+    environment:
+      TEST_DATABASE_URL: postgresql+asyncpg://reactbin:reactbin@postgres-test:5432/reactbin_test
+      DATABASE_URL: postgresql+asyncpg://reactbin:reactbin@postgres-test:5432/reactbin_test
+      S3_ENDPOINT_URL: http://minio-test:9000
+      S3_BUCKET_NAME: reactbin-test
+      S3_ACCESS_KEY_ID: minioadmin
+      S3_SECRET_ACCESS_KEY: minioadmin
+      S3_REGION: us-east-1
+      JWT_SECRET_KEY: test-secret-key-for-testing-only
+      OWNER_USERNAME: testowner
+      OWNER_PASSWORD: testpassword
+      API_BASE_URL: http://localhost:8000
+      MAX_UPLOAD_BYTES: "52428800"
+    depends_on:
+      postgres-test:
+        condition: service_healthy
+      minio-init-test:
+        condition: service_completed_successfully
+    command: ["python", "-m", "pytest", "tests/", "-v"]
+    working_dir: /app