refactor: Add schema-level validation for user fields

[Devexhhh]

Description

This PR adds schema-level validation and sanitization improvements to the User model to improve data integrity, consistency, and backend reliability.

Changes Made

• Added trim validation for string fields

• Added lowercase sanitization for email field

• Added email format validation using regex

• Added minlength and maxlength constraints where applicable

• Added numeric validation constraints for:

  • placementProbability
  • streak
  • totalXP

• Updated password hashing middleware to use modern async middleware pattern compatible with current Mongoose versions

• Added validation error handling for cleaner API responses

Validation Examples Tested

• Invalid email formats
• Short passwords
• Short names
• Negative numeric values
• Successful user creation with valid inputs

Files Modified

• server/models/User.js

Benefits

• Prevents malformed or inconsistent data from being stored
• Improves backend validation reliability
• Enhances API response correctness
• Maintains compatibility with existing application functionality

Screenshots

Closes #34

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