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/batches/bat-016_20260605_ledger-accounting-consolidation/index.md

@@ -1,6 +1,6 @@
 # BAT-016: ledger 회계 책임 단일화 + portfolio_ledger 통합
 
-- Status: verified
+- Status: release-candidate
 - Profile: standard
 - Source Discovery: dcy-006
 - Included REQ: REQ-036, REQ-037, REQ-038