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>

tests/application/test_ledger_outbox.py

@@ -1,7 +1,7 @@
 """LedgerOutbox + LedgerWriter direct unit tests (REQ-019)."""
 from __future__ import annotations
 
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
 
 from trex_core.adapters.ledger.in_memory import InMemoryLedger
 from trex_core.application.ledger_outbox import (
@@ -55,11 +55,16 @@ def test_writer_marks_conflict_blocked_and_does_not_retry():
 
 
 def test_writer_retries_until_success():
-    outbox = InMemoryLedgerOutbox()
+    # dcy-015: retryable rows now back off; share an injected clock between the
+    # outbox (list_pending "now") and the writer (next_retry_at) to advance past it.
+    now = {"t": datetime(2026, 6, 4, tzinfo=timezone.utc)}
+    outbox = InMemoryLedgerOutbox(clock=lambda: now["t"])
     ledger = InMemoryLedger(fail_n_then_succeed=1)
-    writer = LedgerWriter(outbox, ledger)
+    writer = LedgerWriter(outbox, ledger, clock=lambda: now["t"])
     outbox.enqueue(_trade(execution_id="e-r"))
     s1 = writer.run_once()
     assert s1.retryable == 1 and ledger.count() == 0
+    assert writer.run_once().completed == 0  # still in backoff -> skipped
+    now["t"] += timedelta(seconds=600)       # past the backoff window
     s2 = writer.run_once()
     assert s2.completed == 1 and ledger.count() == 1