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>

.env.test.example

@@ -0,0 +1,29 @@
+# Integration test environment variables
+# Used when running pytest directly on the host (outside Docker).
+#
+# Start test services first:
+#   docker compose -f docker-compose.test.yml up -d postgres-test minio-test minio-init-test
+#
+# Then source this file and run tests:
+#   export $(grep -v '^#' .env.test.example | xargs)
+#   cd api && python -m pytest tests/integration/ -v
+
+# PostgreSQL test database (postgres-test container on host port 5433)
+TEST_DATABASE_URL=postgresql+asyncpg://reactbin:reactbin@localhost:5433/reactbin_test
+DATABASE_URL=postgresql+asyncpg://reactbin:reactbin@localhost:5433/reactbin_test
+
+# MinIO test instance (minio-test container on host port 9002)
+S3_ENDPOINT_URL=http://localhost:9002
+S3_BUCKET_NAME=reactbin-test
+S3_ACCESS_KEY_ID=minioadmin
+S3_SECRET_ACCESS_KEY=minioadmin
+S3_REGION=us-east-1
+
+# Auth (test values — not for production)
+JWT_SECRET_KEY=test-secret-key-for-testing-only
+OWNER_USERNAME=testowner
+OWNER_PASSWORD=testpassword
+
+# API
+API_BASE_URL=http://localhost:8000
+MAX_UPLOAD_BYTES=52428800