Implement IProtectedUserStore for automatic personal data encryption

[Copilot]

This PR implements the IProtectedUserStore functionality requested in the TODO comment by adding automatic encryption/decryption for user personal data marked with [ProtectedPersonalData].

Changes Made

1. PersonalDataProtectionService

• Created Services/PersonalDataProtectionService.cs implementing IPersonalDataProtector
• Uses ASP.NET Core Data Protection API for secure encryption/decryption
• Includes backward compatibility for existing unencrypted data
• Handles null/empty values gracefully

2. Identity Configuration Updates

• Enabled options.Stores.ProtectPersonalData = true in Identity setup
• Added Data Protection services registration
• Registered PersonalDataProtectionService as IPersonalDataProtector implementation
• Removed the TODO comment indicating completion

3. Automatic Protection for Existing Fields

The existing user model already has the required attributes:

public class EssentialCSharpWebUser : IdentityUser
{
    [ProtectedPersonalData]  // Will now be automatically encrypted
    public virtual string? FirstName { get; set; }
    
    [ProtectedPersonalData]  // Will now be automatically encrypted  
    public virtual string? LastName { get; set; }
    
    public int ReferralCount { get; set; }  // Not encrypted
}

How It Works

• Encryption: When user data is saved, properties with [ProtectedPersonalData] are automatically encrypted
• Decryption: When user data is loaded, encrypted properties are automatically decrypted
• Backward Compatibility: Existing unencrypted data continues to work unchanged
• Security: Uses ASP.NET Core's Data Protection API with proper key management and rotation

Testing

Added comprehensive tests covering:

• Encryption/decryption functionality
• Null and empty string handling
• Special character support
• Backward compatibility with existing data
• Integration with the existing user model

Migration Impact

• ✅ No database migration required
• ✅ No breaking changes
• ✅ Existing unencrypted data continues to work
• ✅ New data will be automatically encrypted

Fixes #251.

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