Add schema validation to request payloads

[fuzziecoder]

Title

feat: add schema-based request validation for backend APIs
 
Body
Validation is currently minimal. Add schema validation for login and order payloads to reject malformed data consistently.
 
Acceptance Criteria

• Invalid payloads return structured 422 responses.
• Validation errors include field-level messages.
• All POST endpoints use a common validation pattern.
   

[srija-pixel]

hi can you assign this to me?

[srija-pixel]

hi please can you assign this