feat(email): add provider abstraction and HTML templates

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.

gatehouse_app/services/external_auth/linking.py

@@ -136,7 +136,7 @@ def complete_link_flow(
     ).first()
     if conflicting:
         raise ExternalAuthError(
-            f"This {provider_type_str} account is already linked to a different Gatehouse user.",
+            f"This {provider_type_str} account is already linked to a different Secuird user.",
             "PROVIDER_ALREADY_LINKED",
             409,
         )
@@ -246,10 +246,10 @@ def authenticate_with_provider(
             provider_user_id=user_info["provider_user_id"],
             email=user_info["email"],
             failure_reason="account_not_found",
-            error_message="No Gatehouse account matches this external account",
+            error_message="No Secuird account matches this external account",
         )
         raise ExternalAuthError(
-            "No Gatehouse account matches this external account. Please register first.",
+            "No Secuird account matches this external account. Please register first.",
             "ACCOUNT_NOT_FOUND",
             400,
         )