feat: IMAP STATUS, SUBSCRIBE/LSUB, real INTERNALDATE

[Jaro-c]

Problem

Three gaps blocking real IMAP client compatibility (part of #81):

1. No STATUS command — every MUA polls STATUS before selecting; without it clients can't detect new mail without re-SELECTing.
2. No SUBSCRIBE/UNSUBSCRIBE/LSUB — legacy clients (Thunderbird default, mutt, iOS Mail) rely on LSUB to enumerate mailboxes; LIST alone is insufficient.
3. INTERNALDATE returns 1970-01-01 — clients use INTERNALDATE for message sort and display; the epoch placeholder breaks chronological ordering.

Fix

• STATUS handler computes MESSAGES, RECENT, UIDNEXT, UIDVALIDITY, UNSEEN from a snapshot without requiring selection.
• Subscriptions persisted to accounts/<name>/.subscriptions. INBOX always subscribed.
• MessageRef stores file mtime; FETCH INTERNALDATE returns actual delivery time in RFC 3501 format.

Tests

• status_reports_counts_for_inbox — counts correct before/after STORE
• status_requires_authentication_and_existing_mailbox — auth + existence guards
• subscribe_and_lsub_flow — subscribe/unsubscribe/lsub round-trip
• internaldate_is_not_epoch — ensures epoch placeholder is gone
• internaldate_format_sanity — verifies date arithmetic

[Jaro-c]

Fixed by PR #101: STATUS, SUBSCRIBE/LSUB, and real INTERNALDATE implemented.