Use dependency injection

Use FastAPI's Depends() for dependency injection:
- Database sessions
- Authentication
- Configuration
- External services

```python
from fastapi import Depends
from sqlalchemy.ext.asyncio import AsyncSession

async def get_db() -> AsyncGenerator[AsyncSession, None]:
async with async_session() as session:
yield session

async def get_current_user(
token: str = Depends(oauth2_scheme),
db: AsyncSession = Depends(get_db)
) -> User:
...

@app.get("/me")
async def get_me(user: User = Depends(get_current_user)):
return user
```