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.example

@@ -5,4 +5,16 @@
 JWT_SECRET=your-secure-jwt-secret-key-here
 
 # PostgreSQL database password
-DBPASS=your-secure-database-password-here
+DBPASS=your-secure-database-password-here
+
+# Test Mode Configuration (WARNING: Only set to true for integration testing)
+# When enabled, rate limiting is disabled to allow rapid test execution
+# Never enable in production environments
+TEST_MODE=false
+
+# Rate Limiting Configuration
+# Set to false to disable rate limiting (only for test environments)
+RATE_LIMIT_ENABLED=true
+# Number of requests allowed per minute per IP (default: 10)
+# Increase for integration testing (e.g., 1000)
+REQUESTS_PER_MINUTE=10