Update go.mod module declaration and all internal imports across the backend codebase to use simplified nexavpn/backend path instead of full GitHub URL.
Extract Claims struct from auth/types.go into dedicated identity package for better separation of concerns. Update all imports and usages across auth service, token handling, and request context utilities.
Move ClaimsFromContext and MustUserID helpers from httpserver to new requestctx package for better separation of concerns. Update all imports across auth, device, policy, and user handlers. Fix Dockerfile to copy go.sum and run go mod tidy before download.
