Subsystem to monitor IMAP mailbox for new messages.
Introduces:
- intake: worker that uses IDLE or polling to detect new emails.
- imap: client wrapper for connection management and IMAP commands.
- filter: logic for IMAP search and sender allow-list.
- tracker: concurrency control to prevent processing the same UID twice.
- backoff: for handling connection retries with jitter.
c53ee5f6ad
