chore: add centralized request validation using Zod

[devxsameer]

Summary

Introduces centralized request validation using Zod for API inputs.

Changes

• Added reusable validation middleware (validate.ts)
• Added schemas for params and query (schemas.ts)
• Applied validation to streams endpoints
• Ensures consistent 400 error shape: { error, details }

Tests

• Covered invalid UUID, query params, and enum values
• All tests passing

Security Notes

• Eliminates reliance on unchecked req.body / req.query
• Prevents malformed input from reaching business logic
  closes Request validation with Zod (or Joi) for all write routes #7

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