feat(http): Implement HTTP request body size limit middleware

[allisson]

Add a global middleware to limit the size of incoming request bodies,
improving system resilience against Denial-of-Service (DoS) attacks.

Changes:

• Add MaxRequestBodySize configuration (default 1MB) via MAX_REQUEST_BODY_SIZE environment variable.
• Implement MaxRequestBodySizeMiddleware in internal/http using http.MaxBytesReader.
• Wrap request body to intercept http.MaxBytesError and return 413 Payload Too Large.
• Integrate the middleware into the global Gin router in SetupRouter.
• Update integration test helpers to include the new limit in test configurations.
• Document the new security feature in conductor/tech-stack.md.