fix: ledger write idempotency for TimescaleDB hypertable (BAT-016 rework)

BAT-016's initial implementation passed only against mocked repositories and
broke on a real database. Two defects fixed:

1. insert_ledger_trade used `ON CONFLICT (portfolio_id, order_id, execution_id)
   WHERE order_id IS NOT NULL` but portfolio_ledger is a TimescaleDB hypertable,
   which cannot have that partial unique index (must include the partitioning
   column recorded_at). The statement errored against Postgres. Replaced with
   application-level idempotency: SELECT-before-insert inside the existing
   `SELECT ... FOR UPDATE` portfolio row lock, which serializes concurrent
   same-portfolio writes (race-safe). Duplicate returns current authoritative
   portfolio state (dcy-006 OQ-4 b).

2. `with self._connect() as conn:` closes the shared psycopg3 connection on exit,
   leaving the reused repository connection unusable. Switched to
   `with conn.transaction():` (commits without closing).

Added tests/test_ledger_consolidation_integration.py — real-Postgres integration
tests (new / duplicate / negative-balance) that close the mock-only verification
gap. Full suite: 376 passed.

SDLC: REQ-036/037 CHG-20260605-03 reconcile OQ-7 (ii) (DB partial unique →
application-level idempotency) since the hypertable makes the DB constraint
infeasible. REQ-036/037/038 → Implemented; BAT-016 → release-candidate.

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

docs/srs/Interface/req-038_legacy-buy-sell-endpoint-deprecation.md

@@ -1,6 +1,6 @@
 # REQ-038: 레거시 buy/sell endpoint deprecated 표기
 
-- Status: Approved
+- Status: Implemented
 - Type: Interface
 - Priority: Low
 - Owner: Justin Ko
@@ -39,6 +39,34 @@
 
 ## Change Log
 
+### CHG-20260605-03
+- Date: 2026-06-05
+- Author: Justin Ko
+
+#### Change Summary
+- Status: Approved → Implemented
+
+#### Intent
+- BAT-016 confirm-batch-verification 완료. 레거시 buy/sell `deprecated=True` + `Deprecation: true` 헤더 구현, 동작 보존. AC-1~4 검증(`tests/test_auth.py::test_legacy_buy_sell_routes_emit_deprecation_header` 포함). 전체 376 passed.
+
+#### Acceptance Criteria Delta
+- No change
+
+#### Impacted Area Delta
+- No change
+
+#### Delivery Trace Delta
+- Batches:
+  - BAT-016 (release-candidate)
+- Releases:
+  - REL-003 (예정)
+
+#### Revalidation Impact
+- Reverification Needed: no
+- Existing Implementation Invalidated: no
+- Status Recommendation: implemented
+- Follow-up Action: 실제 endpoint 제거는 후속 정리 트랙
+
 ### CHG-20260605-02
 - Date: 2026-06-05
 - Author: Justin Ko