ADR 0359 decided one module holds the operator's mail accounts. This is its
first stage: an imap adapter that keeps IDLE on the inbox and polls the other
folders read-only, indexes each message in its own database, keeps each
attachment once by its SHA-256 in its bucket, and says every added, changed or
removed message as a metadata-only event. Bodies and attachments are served
only by its tools, to callers the trusted setting grants names. Its health
waits for the account's own secret until it is given at the desk (issue 710).
