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app/api/deps.py

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+"""Shared FastAPI dependencies — the composition root for request-scoped wiring.
+
+Concrete adapters are bound to ports here so routers and services stay
+decoupled from infrastructure. Repository/service providers are added in
+later steps as the domain grows.
+"""
+
+from collections.abc import AsyncIterator
+from typing import Annotated
+
+from fastapi import Depends
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.infrastructure.db import get_sessionmaker
+
+
+async def get_session() -> AsyncIterator[AsyncSession]:
+    """Request-scoped DB session. Commits on success, rolls back on exception."""
+    session = get_sessionmaker()()
+    try:
+        yield session
+        await session.commit()
+    except Exception:
+        await session.rollback()
+        raise
+    finally:
+        await session.close()
+
+
+SessionDep = Annotated[AsyncSession, Depends(get_session)]