feat(queue): dead-letter store for group shared queues (§11.6 D3 / Track A M2)

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>

crates/manager-core/tests/expected_schema.txt

@@ -231,6 +231,24 @@ table: group_collections
   created_at: timestamp with time zone NOT NULL default=now()
   updated_at: timestamp with time zone NOT NULL default=now()
 
+table: group_dead_letters
+  id: uuid NOT NULL default=gen_random_uuid()
+  group_id: uuid NOT NULL
+  collection: text NOT NULL
+  original_event_id: uuid NOT NULL
+  source: text NOT NULL
+  op: text NOT NULL
+  trigger_id: uuid NULL
+  script_id: uuid NULL
+  payload: jsonb NOT NULL
+  attempt_count: integer NOT NULL
+  first_attempt_at: timestamp with time zone NOT NULL
+  last_attempt_at: timestamp with time zone NOT NULL
+  last_error: text NOT NULL
+  created_at: timestamp with time zone NOT NULL default=now()
+  resolved_at: timestamp with time zone NULL
+  resolution: text NULL
+
 table: group_docs
   group_id: uuid NOT NULL
   collection: text NOT NULL
@@ -567,6 +585,10 @@ indexes on group_collections:
   group_collections_group_uidx: public.group_collections USING btree (group_id, lower(name), kind) WHERE (group_id IS NOT NULL)
   group_collections_pkey: public.group_collections USING btree (id)
 
+indexes on group_dead_letters:
+  group_dead_letters_pkey: public.group_dead_letters USING btree (id)
+  idx_group_dead_letters_group: public.group_dead_letters USING btree (group_id, created_at DESC)
+
 indexes on group_docs:
   group_docs_pkey: public.group_docs USING btree (group_id, collection, id)
   idx_group_docs_data_gin: public.group_docs USING gin (data jsonb_path_ops)
@@ -814,6 +836,11 @@ constraints on group_collections:
   [FOREIGN KEY] group_collections_group_id_fkey: FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE
   [PRIMARY KEY] group_collections_pkey: PRIMARY KEY (id)
 
+constraints on group_dead_letters:
+  [CHECK] group_dead_letters_resolution_check: CHECK ((resolution = ANY (ARRAY['replayed'::text, 'ignored'::text, 'handled_by_script'::text, 'handler_failed'::text])))
+  [FOREIGN KEY] group_dead_letters_group_id_fkey: FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE
+  [PRIMARY KEY] group_dead_letters_pkey: PRIMARY KEY (id)
+
 constraints on group_docs:
   [FOREIGN KEY] group_docs_group_id_fkey: FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE
   [PRIMARY KEY] group_docs_pkey: PRIMARY KEY (group_id, collection, id)
@@ -1005,3 +1032,4 @@ constraints on vars:
   0065: group queues
   0066: projects
   0067: project environments
+  0068: group dead letters