Passive Group-scoped overdue Mail summary #179

On a Group's Mail view, an operator can currently see unread Mail per Actor but has no single read that answers "which Actors still have direct Mail awaiting them past an age cutoff, how many, and where is the oldest?"

The proposed primitive is one Group-scoped read, mail_overdue_summary, taking group_id and required positive after_seconds, returning per-Actor awaiting_count, overdue_count, oldest_overdue_at, oldest_source_event_id, unaged_count, plus evaluated_at. It reuses the existing awaiting predicate (read cursor, reply, accepted/ambiguous delivery) and NoticeState source resolution, so "awaiting" means the same thing as production notices. It does not deliver, page, or change state. Overdue age starts at the Mail source timestamp. Rows are only Actors with an overdue or unaged item; all current non-internal Actors including stopped/disabled are eligible. Access follows existing Group transport and inbox authorization; a peer's own-inbox permission does not grant other Actors' rows. No background sampler, no new permission, no cross-Group aggregation.

Thank you to the maintainer for the #166 direction (passive Group observation) and #165 (production NoticeState path). Related: #162, #171. We will not ask cccc to implement escalation, reminders, or foreman policy in core.

How we checked

On our staging instance (cccc 0.4.44; KVM guest, 8 vCPU Intel Skylake, 15 GiB RAM, Debian 13), with real Group data and a 900-second cutoff:

• Three real direct Mail items were unread, unreplied and had no accepted or ambiguous delivery. They were 7 to 11 days old, and one was addressed to a stopped Actor.
• inbox_peek lists them one recipient at a time. The Mail view shows unread badges and a filter.
• Nothing gives the Group-level answer: there is no caller cutoff, no per-Actor overdue count and no oldest item.
• Nothing was sent or read during the check.