fix: isolated thread에 독립적인 DB 엔진 생성

asyncpg 커넥션은 event loop에 바인딩됨. API의 AsyncSessionLocal을
다른 loop에서 사용하면 'Future attached to a different loop' 오류 발생.
→ build thread 내에서 전용 engine + session factory 생성.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/api/v1/endpoints/universe.py

@@ -196,10 +196,33 @@ async def _run_build_snapshots(
         _asyncio.set_event_loop(loop)
 
         async def _build():
+            # Create a fresh engine + session factory bound to THIS event loop.
+            # asyncpg connections are loop-bound; reusing the API's AsyncSessionLocal
+            # in a different loop raises "Future attached to a different loop".
+            from sqlalchemy.ext.asyncio import (
+                AsyncSession,
+                async_sessionmaker,
+                create_async_engine,
+            )
+            from sqlalchemy.orm import sessionmaker as _sessionmaker
+            from app.core.config import settings
+
+            _engine = create_async_engine(
+                settings.DATABASE_URL,
+                echo=False,
+                pool_size=2,
+                max_overflow=0,
+                pool_timeout=30,
+                pool_pre_ping=True,
+                connect_args={"server_settings": {"jit": "off"}},
+            )
+            _SessionLocal = _sessionmaker(
+                _engine, class_=AsyncSession, expire_on_commit=False
+            )
             svc = UniverseService()
             try:
                 result = await svc.build_snapshots(
-                    AsyncSessionLocal,
+                    _SessionLocal,
                     tickers=tickers,
                     start_date=start_date,
                     end_date=end_date,
@@ -208,6 +231,8 @@ async def _run_build_snapshots(
                 logger.info(f"Universe background build complete: {result}")
             except Exception as e:
                 logger.error(f"Universe background build failed: {e}")
+            finally:
+                await _engine.dispose()
 
         try:
             loop.run_until_complete(_build())