test: add test tooling and documentation

- Add Makefile with convenient test targets (test, test-integration, test-env-up, test-env-down)
- Add .env.test with test-specific configuration
- Update .env.example with test configuration options and warnings
- Update README.md with comprehensive testing documentation
- Document all environment variables with safety warnings

This makes it easy to run tests without rate limiting issues while
keeping production security intact.

.env.test

@@ -0,0 +1,25 @@
+# Test environment configuration
+# WARNING: This configuration disables security features for testing only
+# Never use these settings in production
+
+# Database configuration
+DBPASS=027b61803f1d6d1873b934e8
+DATABASE_HOST=localhost
+DATABASE_PORT=5432
+DATABASE_USER=postgres
+DATABASE_PASSWORD=027b61803f1d6d1873b934e8
+DATABASE_NAME=ebookdb
+
+# JWT configuration
+JWT_SECRET=f3b6693578fdbd0671adb5c9971e1f8b
+
+# Server configuration
+SERVER_PORT=8080
+
+# Upload path
+UPLOAD_PATH=./uploads
+
+# Test configuration (for integration testing)
+TEST_MODE=true
+RATE_LIMIT_ENABLED=false
+REQUESTS_PER_MINUTE=1000