#1002 Stale smtp_password refs (shared/mail) in 10 welcome_email/invitation_email installations; tenant app secret DELETE cannot remove a ref without a value

EARS: Installations shall not reference retired platform secrets; DELETE /api/v1/tenant/apps/<installation>/secrets/<name> shall remove the reference even when no stored value exists. FOUND in #924 (read-only forensic query, application_name hw924-forensic-readonly): 10 enabled installations (platform.welcome_email and platform.invitation_email in tenants demo, _platform, platform, strictprobe, test) map smtp_password to vault:shared/mail:highway-mail, the SMTP password retired by #968. The apps list no required secrets and do not read it. After #924 the ref becomes secret:shared/mail:highway-mail, the executor refuses it (not in APP_PLATFORM_SECRETS) and logs a warning per run; before #924 Vault handed the dead password to the apps. tenant_apps_secrets.delete_secret returns 404 when the value is missing and leaves the ref, so there is no API path to clean them.