Re-run connector post-install steps

Re-runs the connector's declared post-install steps (identity resolution, provider-side setup) against this connection. Use it to repair post_install_error, or to re-apply setup after a variable was corrected. No body. Same permission as reconnect; the detail read's can_run_post_install says whether to offer it. Answers 202 { "started": true }, never 200: the steps run in the background, so the real outcome lands on the row (status and last_error) and shows on a later read. Re-fetch on the user's own refresh rather than sleeping and treating the 202 as the result. 409 when the connector declares no steps (no_post_install), the connection is unbound (not_bound), it is already fully set up (already_active; this route repairs post_install_error, not a healthy connection), its sign-in needs to happen again (needs_reauth), or Elaichi no longer holds the account (connection_disconnected).