When a user accepts an org invite, send a notification email to the
person who sent the invite with membership details (member name, email,
org name, role) and an optional View Organization button.
Added build_invite_accepted_html() template to email_templates.py,
wired it into the accept_invite() handler, and added a test case.
Add pluggable email provider system supporting SMTP, Mailgun, and SendGrid
with factory pattern for runtime provider selection. Includes branded HTML
email templates for verification, password reset, MFA notifications, and
organization invites.
Also rebrands all email content from Gatehouse to Secuird, adds email
provider configuration options, and fixes duplicate log handlers in
development mode.