fix(ledger): dcy-015 outbox 영속 오류 분류 + 백오프/dead-letter (BAT-019)

portfolio-hub 404(미등록 포트폴리오)가 retryable 로 분류되어 백오프·상한 없이
초당 영구 재시도되던 버그(issue #13, 실측 930회) 정정.

거버넌스: 전부 change-req (신규 REQ 없음, dcy-015 OQ-3; 백오프 env 는 REQ-019 흡수).
- REQ-017: 404/400/422 → LedgerPermanentError 분류 (409 conflict / 408·429·5xx·
  network retryable 불변). LedgerPort 에 permanent 타입 추가
- REQ-019: 영속 → 터미널 delivery_failed 즉시(재시도 0). retryable 지수 백오프
  +jitter+next_retry_at, max_attempts 초과 시 dead-letter delivery_failed.
  list_pending 이 백오프 행 스킵(head-of-line 보호). 마이그레이션 007(status enum
  +next_retry_at). 백오프 env(base/cap/max-attempts)
- REQ-018: 터미널/dead-letter WARN + WriterStats.delivery_failed 카운터 가시화
- REQ-020: runtime-flows/data-model/interface-contract 정합

단일 터미널 delivery_failed(permanent/dead-letter, last_outcome 로 사유 구분) —
conflict_blocked 패턴과 동형. dead-letter 는 운영자 재처리 가능 상태.

검증: 단위 311 passed/34 skipped, 인프라 게이트(PG+RabbitMQ) 345 passed, ruff clean.
신규 test_ledger_outbox_backoff + adapter/PG 테스트, 기존 retryable 백오프 정합.
BAT-019 release-candidate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

trex_core/adapters/persistence/sql/007_ledger_outbox_delivery_failed_and_backoff.sql

@@ -0,0 +1,25 @@
+-- BAT-019 / dcy-015 / REQ-019 (issue #13): ledger_outbox terminal failure +
+-- backoff scheduling. Adds:
+--   - next_retry_at: earliest re-attempt time for a retryable row (backoff).
+--   - status 'delivery_failed': single terminal for permanent (404/400/422)
+--     and dead-lettered (max_attempts exhausted) rows; reason in last_outcome.
+--   - last_outcome 'permanent_failure'/'dead_lettered'.
+-- Idempotent: ADD COLUMN IF NOT EXISTS + DROP/ADD CONSTRAINT (re-runnable since
+-- migrations are re-applied by glob).
+ALTER TABLE ledger_outbox ADD COLUMN IF NOT EXISTS next_retry_at TIMESTAMPTZ;
+
+ALTER TABLE ledger_outbox DROP CONSTRAINT IF EXISTS ledger_outbox_status_check;
+ALTER TABLE ledger_outbox ADD CONSTRAINT ledger_outbox_status_check
+    CHECK (status IN ('pending','completed','conflict_blocked','delivery_failed'));
+
+ALTER TABLE ledger_outbox DROP CONSTRAINT IF EXISTS ledger_outbox_last_outcome_check;
+ALTER TABLE ledger_outbox ADD CONSTRAINT ledger_outbox_last_outcome_check
+    CHECK (last_outcome IS NULL OR last_outcome IN (
+        'completed','duplicate_completed','retryable_failure','conflict_blocked',
+        'permanent_failure','dead_lettered'
+    ));
+
+-- Partial index so the writer's pending scan skips rows still in backoff cheaply.
+CREATE INDEX IF NOT EXISTS idx_ledger_outbox_pending_retry
+    ON ledger_outbox (next_retry_at)
+    WHERE status = 'pending';