[Server] Expand Request Body Validation with Zod

[Pushkarini579]

Integrate the zod library across all server routes (auth, analysis, history). Ensure the server returns a clear 400 Bad Request if validation fails. Note: A basic schema has already been added to server/index.js for patientName.

[deepikavashishtha665]

Hello @Pushkarini579
I am contributing under GSSoC'26. I would like to work on this issue. please assign it to me.
/assign me under gssoc