Improve WellAI email verification

[Sarasaidin]

Summary

Improved the WellAI email verification system with branded email content and reliable inline logo embedding.

Changes Made

• Updated the verification email sender display name to WellAI.
• Updated the email subject to Verify your WellAI account.
• Improved the WellAI verification email design and branding.
• Added the WellAI logo as an inline CID image so it displays correctly in email clients.
• Added the MIMEImage handling required for embedded email images.
• Added the required Path import for locating the WellAI logo.
• Maintained the existing email verification and validation flow.
• Tested registration, email delivery, verification, account deletion, and re-registration using the same email address.

Testing

• Verified that the verification email is successfully sent through Brevo.
• Confirmed the email reaches Gmail Inbox.
• Confirmed the WellAI logo displays correctly.
• Confirmed the verification button works.
• Confirmed email verification works.
• Confirmed an Admin can delete the account.
• Confirmed the same email can register again after deletion.
• Confirmed a new verification email is sent after re-registration.

Files Changed

• server/routers/authentication.py
• server/utils/email_service.py

No DNS configuration files or credentials are included in this PR.