fix(migrations): R1 baseline 런타임 버그 3건 — init_db asyncpg 경로 (R1 fix)

★실제 init_db() 런타임 검증(psql migration_smoke 가 못 잡는 asyncpg 경로)에서 발견·수정:
1. baseline 덤프에 CREATE TABLE schema_migrations 포함 → init_db 가 IF NOT EXISTS 로 선-CREATE
   후 baseline 이 재-CREATE 충돌. --exclude-table=schema_migrations 재덤프(init_db 가 소유).
2. baseline 은 multi-statement 인데 exec_driver_sql(asyncpg prepared)은 multi-statement 불허
   ('cannot insert multiple commands into a prepared statement'). raw asyncpg simple 프로토콜
   execute() 로 적재(같은 connection = 트랜잭션 내).
3. 마이그 360(10 DROP)·361(DELETE+CREATE)이 multi-statement → init_db 적용 실패. 360=콤마구분
   단일 DROP, 361=단일 CREATE UNIQUE INDEX(prod 중복0·fresh 빈테이블이라 dedup DELETE 불요).

★검증: scripts/ci/initdb_runtime_test.py 로 실제 init_db 2회 — 1st(fresh: baseline 262 스탬프 +
359/360/361 적용, documents·purge_col·cand_drop·attempt_unique 전부 확인), 2nd(멱등 skip) PASS.
psql migration_smoke 도 PASS 유지.

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

scripts/ci/initdb_runtime_test.py

@@ -0,0 +1,51 @@
+"""init_db() baseline 부팅 런타임 검증 (R1) — psql migration_smoke 가 못 잡는 asyncpg 경로 확인.
+
+migration_smoke.sh(psql)는 SQL 유효성만 검증한다. init_db 는 asyncpg exec_driver_sql(prepared)
+경로라 ① multi-statement 불허 ② baseline 의 raw asyncpg 적재 ③ skip/stamp/멱등 — 이걸 실측한다.
+
+실행 (worktree 루트):
+  python3.11 -m venv /tmp/v && /tmp/v/bin/pip install -q "sqlalchemy[asyncio]>=2" asyncpg pydantic pyyaml
+  docker run -d --name idb -p 55432:5432 -e POSTGRES_HOST_AUTH_METHOD=trust pgvector/pgvector:pg16
+  docker exec idb psql -U postgres -c "CREATE DATABASE pkm"
+  ln -sfn ../migrations app/migrations   # Docker 의 /app/migrations 레이아웃 모사 (테스트 후 rm)
+  PYTHONPATH=app DATABASE_URL="postgresql+asyncpg://postgres@localhost:55432/pkm" /tmp/v/bin/python scripts/ci/initdb_runtime_test.py
+  rm -f app/migrations; docker rm -f idb
+
+기대: 1st OK(documents=True·purge_col=1·cand_qwen=0·attempt_unique=1), 2nd 멱등동일=True.
+"""
+import asyncio
+from sqlalchemy import text
+
+
+async def main():
+    from core.config import settings
+    url = settings.database_url
+    print("effective DATABASE_URL:", url)
+    assert "localhost" in url or "127.0.0.1" in url, f"SAFETY ABORT non-local: {url}"
+    from core.database import init_db, async_session, engine
+
+    print("=== 1st init_db (fresh DB) ===")
+    await init_db()
+    async with async_session() as s:
+        cnt = (await s.execute(text("SELECT count(*) FROM schema_migrations"))).scalar()
+        mx = (await s.execute(text("SELECT max(version) FROM schema_migrations"))).scalar()
+        bl = (await s.execute(text("SELECT count(*) FROM schema_migrations WHERE name LIKE 'baseline:%'"))).scalar()
+        docs = (await s.execute(text("SELECT to_regclass('public.documents') IS NOT NULL"))).scalar()
+        purge = (await s.execute(text("SELECT count(*) FROM information_schema.columns WHERE table_name='documents' AND column_name='purge_requested_at'"))).scalar()
+        cand = (await s.execute(text("SELECT count(*) FROM information_schema.tables WHERE table_name LIKE 'documents_cand_qwen%'"))).scalar()
+        uq = (await s.execute(text("SELECT count(*) FROM pg_indexes WHERE indexname='uq_attempt_session_question'"))).scalar()
+    print(f"  schema_migrations count={cnt} max={mx} baseline_stamped={bl}")
+    print(f"  documents={docs} purge_col={purge} cand_qwen_tables={cand} attempt_unique={uq}")
+    assert docs and purge == 1 and cand == 0 and uq == 1, "FAIL: 기대 스키마 상태 불일치"
+
+    print("=== 2nd init_db (rerun = baseline skip + 멱등) ===")
+    await init_db()
+    async with async_session() as s:
+        cnt2 = (await s.execute(text("SELECT count(*) FROM schema_migrations"))).scalar()
+    assert cnt == cnt2, "FAIL: 멱등 아님 (재실행이 schema_migrations 변경)"
+    print(f"  count={cnt2} 멱등동일={cnt == cnt2}")
+    print("RESULT: PASS — init_db baseline 부팅/멱등 검증")
+    await engine.dispose()
+
+
+asyncio.run(main())