IdP Subject Token Storage (removed)

The server edition no longer persists identity-provider access or refresh tokens after login. The feature existed only to feed an on-behalf-of token exchange that was never wired to any upstream call, so a long-lived IdP refresh token at rest had no reader and was a leak surface rather than a capability. server_edition.store_idp_tokens is still accepted by the config loader for compatibility, but it is a no-op: a value of true logs one deprecation warning at startup (server_edition.store_idp_tokens is deprecated and no longer stores IdP tokens; remove it) and nothing is stored. Tokens an earlier release persisted are deleted from config.db on the first start with server_edition.enabled: true after upgrading, before any other server-edition setup step and without needing an encryption key (logged as purged legacy IdP subject-token rows); nothing in the current release could read them anyway. Per-upstream credentials obtained through the oauth_connect flow are unaffected — they are still stored encrypted under credential_encryption_key / MCPPROXY_CRED_KEY, as described in Auth Broker and Credential Commands.