Add username field to User model

[3nd3r1]

Add a username field to the User model to support displaying usernames in addition to email.

Technical Details:

• Update app/models/user.py to add username field
• Create Alembic migration for database schema change
• Make username unique and required
• Update existing auth endpoints to accept username during registration
• Update UserResponse schema to include username

Acceptance Criteria:

• User model has username field
• Database migration runs successfully
• Username is unique constraint in database
• Registration endpoint accepts and stores username
• /api/v1/users/me endpoint returns username