feat: add configurable test mode and rate limiting

- Add TestMode, RateLimitEnabled, RequestsPerMinute to Config
- Add getEnvBool() and getEnvInt() helper functions
- Update rate limiter to support enabled/disabled state
- Pass test environment variables through docker-compose
- Configure rate limiter dynamically in main.go

This allows disabling rate limiting for integration testing while
maintaining security in production environments.

docker-compose.yml

@@ -34,6 +34,9 @@ services:
       DATABASE_NAME: bookmann
       JWT_SECRET: ${JWT_SECRET}
       SERVER_PORT: 8765
+      TEST_MODE: ${TEST_MODE:-false}
+      RATE_LIMIT_ENABLED: ${RATE_LIMIT_ENABLED:-true}
+      REQUESTS_PER_MINUTE: ${REQUESTS_PER_MINUTE:-10}
     ports:
       - "8765:8765"
     depends_on: