Backend: added support for stateless chat history in /chat endpoint

Fix: correct CORS configuration (disabled credentials for wildcard origins) to resolve connectivity issues in browsers

Comprehensive fix: systematically migrated all ChatGroq calls to the modern .invoke() syntax across the entire backend

Fix ChatGroq invocation syntax (use .invoke instead of callable)

Fix CORS headers for mobile browser compatibility

Fix LangChain imports for cloud deployment

Initial clean deployment (no binaries)