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/api/v1/organizations/invites.py

@@ -39,12 +39,12 @@ def create_org_invite(org_id):
 
     NotificationService._send_email_async(
         to_address=email,
-        subject=f"You're invited to join {org.name} on Gatehouse",
+        subject=f"You're invited to join {org.name} on Secuird",
         body=(
-            f"You've been invited to join {org.name} on Gatehouse.\n\n"
+            f"You've been invited to join {org.name} on Secuird.\n\n"
             f"Click the link below to accept the invitation (valid for 7 days):\n"
             f"{invite_link}\n\n"
-            f"Gatehouse Security Team"
+            f"Secuird Security Team"
         ),
     )
     logging.getLogger(__name__).info(f"[INVITE] Email queued for {email}")

gatehouse_app/api/v1/organizations/members.py

@@ -167,9 +167,9 @@ def send_mfa_reminder(org_id, user_id):
             body=(
                 f"Hi {user.full_name or user.email},\n\n"
                 "Your organization administrator has asked you to set up "
-                "multi-factor authentication (MFA) on your Gatehouse account.\n\n"
+                "multi-factor authentication (MFA) on your Secuird account.\n\n"
                 "Please log in and configure MFA as soon as possible.\n\n"
-                "Gatehouse Security Team"
+                "Secuird Security Team"
             ),
         )