feat(organizations): email inviter when membership invite is accepted

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.

gatehouse_app/api/v1/organizations/invites.py

@@ -243,6 +243,30 @@ def accept_invite(token):
 
     invite.accept()
 
+    if invite.invited_by and invite.invited_by.email:
+        from gatehouse_app.services.email_templates import build_invite_accepted_html
+        from gatehouse_app.services.notification_service import NotificationService
+
+        member_display = user.full_name or user.email
+        inviter_display = invite.invited_by.full_name or invite.invited_by.email
+        org_link = f"{current_app.config.get('APP_URL', '')}/organizations/{invite.organization_id}"
+
+        html_body = build_invite_accepted_html(
+            inviter_name=inviter_display,
+            member_name=member_display,
+            member_email=user.email,
+            org_name=invite.organization.name,
+            role=invite.role,
+            org_link=org_link,
+        )
+
+        NotificationService._send_email_async(
+            to_address=invite.invited_by.email,
+            subject=f"{member_display} accepted your invitation to {invite.organization.name}",
+            body=f"{member_display} has accepted your invitation to join {invite.organization.name} on Secuird.",
+            html_body=html_body,
+        )
+
     has_webauthn = user.has_webauthn_enabled()
     has_totp = user.has_totp_enabled()