Connect Byo Mailbox

Connect one BYO SMTP+IMAP mailbox — the UI face of the gateway's POST /v1/mailboxes (T-41 PR-3 contract).

The gateway verifies SMTP login + IMAP select BEFORE inserting anything: bad credentials come back as a 422 with a human-readable detail and zero rows on either side — we pass that detail through verbatim. On success the box is upserted into the org's park right away (targeted single-box sync) and the mailbox's domain gets its outreach_organization_domains row so the DNS checker starts covering it — no ops step in the loop.

The body is validated by hand (not as a typed parameter) on purpose: FastAPI's default RequestValidationError echoes the submitted body as the error input — which would put app_password into a 422 response and into anything that captures 4xx bodies. Field names only, never values.