Introduce Zod Validation Middleware Across API Routes

[avishek0769]

Summary

Add a centralized Zod-based request validation layer for required API routes and replace ad-hoc/manual input validation inside controllers.

Problem

Validation is currently inconsistent across routes and controllers. Some controllers perform inline checks, while other inputs are not validated in a uniform way. This increases risk of invalid payloads, duplicated logic, and inconsistent error responses.

Expected Solution

Create reusable Zod schemas and a validation middleware for body, params, and query. Attach middleware at route level for required endpoints, then remove controller-level manual input checks that are replaced by schema validation. Standardize validation error response shape.

Scope

• Backend middleware:
  • backend/middlewares/ (new Zod validation middleware)
• Backend routes:
  • backend/routers/chat.route.js
  • backend/routers/chatMessage.route.js
  • backend/routers/user.route.js
  • backend/routers/apikey.route.js
  • backend/routers/usage.route.js (where applicable)
• Backend controllers (remove replaced manual validation):
  • backend/controllers/chat.controller.js
  • backend/controllers/chatMessage.controller.js
  • backend/controllers/user.controller.js
  • backend/controllers/apikey.controller.js

Acceptance Criteria

• Zod middleware exists and supports validating req.body, req.params, and req.query.
• Required API routes are wired to route-level Zod schemas.
• Manual validation in controllers is removed where equivalent Zod schema validation is in place.
• Validation failures return consistent 4xx responses with clear field-level messages.
• Existing successful request behavior remains backward compatible.