Add user profile endpoints

Extend User Service with profile management endpoints.

Endpoints to Add:

• GET /api/users/{id}/profile
• PUT /api/users/{id}/profile
• POST /api/users/{id}/profile/avatar
• GET /api/users/{id}/preferences
• PUT /api/users/{id}/preferences

Fields:

• Bio, avatar URL, location, website, social links
• User preferences (theme, notifications, language)

Testing:

• Unit tests
• Integration tests
• API documentation updated