fix(invitations): Use tenant-onboard page for platform invitations

Platform/tenant invitations should redirect to /tenant-onboard which has
the full onboarding wizard, not /accept-invite which is for staff invitations.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

smoothschedule/platform_admin/tasks.py

@@ -45,7 +45,7 @@ def send_tenant_invitation_email(self, invitation_id: int):
     try:
         # Build the invitation URL
         base_url = get_base_url()
-        invitation_url = f"{base_url}/accept-invite/{invitation.token}"
+        invitation_url = f"{base_url}/tenant-onboard?token={invitation.token}"
 
         # Email context
         context = {