An exhausted SHARED durable-queue message was `drop_exhausted()`-ed with a
warning — silent data loss. Per-app queues persist to `dead_letters` (0010);
add the symmetric group store so an exhausted shared-queue message is preserved
and operator-visible.
- Migration 0068: `group_dead_letters`, keyed by (group_id, collection),
CASCADE on the group (an app delete leaves the data — it belongs to the
group, not the consuming app).
- `GroupQueueRepo::dead_letter` (replaces `drop_exhausted`): one tx that INSERTs
the dead-letter + DELETEs the live message, filtered by claim_token so a lost
lease can't dead-letter a re-claimed message (mirrors queue_repo::dead_letter).
- Dispatcher `q_terminal` shared arm now dead-letters instead of dropping. It
returns None (not the dl id) so the per-app `fan_out_dead_letter` is SKIPPED —
firing the consuming app's per-app handlers on a shared message (competing
consumers → nondeterministic app) would be wrong. Fan-out to a *shared*
dead_letter trigger is deferred (needs a new trigger kind).
- Read side: `GroupDeadLetterRepo::list_for_group` backs a new read-only
operator endpoint GET /api/v1/admin/groups/{id}/dead-letters (GroupKvRead,
mirrors the M4 group-blobs surface). `pic dead-letters ls --group` deferred
(optional; the HTTP endpoint is the operator surface).
Pinned by group_queue::dead_letter_moves_an_exhausted_message_to_the_group_store
(store move + claim-token-mismatch guard + operator read). Schema golden
reblessed.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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