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app/infrastructure/db/engine.py

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+"""Async engine + session factory, created lazily from settings.
+
+The engine is process-global and cached. The API binds a session per request
+(see ``app.api.deps``); workers and scripts use :func:`session_scope`.
+"""
+
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
+from functools import lru_cache
+
+from sqlalchemy.ext.asyncio import (
+    AsyncEngine,
+    AsyncSession,
+    async_sessionmaker,
+    create_async_engine,
+)
+
+from app.core.config import get_settings
+
+
+@lru_cache
+def get_engine() -> AsyncEngine:
+    settings = get_settings()
+    return create_async_engine(
+        settings.database_url,
+        echo=settings.db_echo,
+        pool_size=settings.db_pool_size,
+        max_overflow=settings.db_max_overflow,
+        pool_pre_ping=True,  # survive Postgres restarts / idle drops
+    )
+
+
+@lru_cache
+def get_sessionmaker() -> async_sessionmaker[AsyncSession]:
+    return async_sessionmaker(
+        bind=get_engine(),
+        expire_on_commit=False,
+        autoflush=False,
+    )
+
+
+@asynccontextmanager
+async def session_scope() -> AsyncIterator[AsyncSession]:
+    """Transactional session for workers/scripts: commit on success, rollback on error."""
+    session = get_sessionmaker()()
+    try:
+        yield session
+        await session.commit()
+    except Exception:
+        await session.rollback()
+        raise
+    finally:
+        await session.close()
+
+
+async def dispose_engine() -> None:
+    """Dispose the pooled engine on shutdown. Safe to call if never initialized."""
+    if get_engine.cache_info().currsize:
+        await get_engine().dispose()
+        get_engine.cache_clear()
+        get_sessionmaker.cache_clear()