perf(slots): BAT-040 — slots RPC DB-level status 필터 (REQ-002/006/073, issue #25)

REQ-002/006 change-req(CHG-20260612-01)→구현→confirm-req-implemented,
REQ-073 change-req(CHG-20260612-02, 관찰 계약 불변 Implemented 유지) 전체 delivery.

- repository.py/in_memory_repository.py: SlotRepository.list_slots(instance_key,
  status_filter=None) — status 값 집합 필터, None=전체(기존 호출자 불변).
- postgresql.py: list_slots DB-level WHERE status = ANY(%s) (full scan + Python
  필터 제거). 스키마/마이그레이션 변경 없음.
- messaging_handlers.py: _FILTER_STATUS 매핑으로 filter→status 집합 위임,
  watchdog 갱신용 holding 조회를 all/holding fetch 결과에서 재사용(2nd 커넥션 생략).
- 관찰 가능한 RPC 계약(REQ-073 AC-1~6) 불변 — 동작 보존 최적화.
- tests: in-memory status_filter 5종 + PG 게이트 1종. 439 passed (+5), ruff clean.

REQ-002·006 Approved→Implemented, REQ-073 Implemented 유지, BAT-040 release-candidate.
issue #25 close.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/batches/index.md

@@ -47,3 +47,4 @@
 | BAT-037 | standard | release-candidate | REQ-073 (CHG-20260612-01) | dcy-025 (REQ-073 change-req) | [bat-037](./bat-037_20260612_slots-rpc-available-slots/) |
 | BAT-038 | standard | release-candidate | REQ-018 (CHG-20260612-01) | dcy-026 (issue #24) | [bat-038](./bat-038_20260612_sell-transaction-tax-fee/) |
 | BAT-039 | standard | release-candidate | REQ-011 (CHG-20260612-01) | 없음 (issue #4) | [bat-039](./bat-039_20260612_paper-seq-redis-persistence/) |
+| BAT-040 | standard | release-candidate | REQ-002 (CHG-20260612-01), REQ-006 (CHG-20260612-01), REQ-073 (CHG-20260612-02) | 없음 (issue #25) | [bat-040](./bat-040_20260612_slots-rpc-db-filter/) |