Pad not removed from backend workers after deletion

[cyclotruc]

When a pad is deleted by a user via the DELETE /pad_id request, there's no logic to remove it from running workers, leading to those warnings in logs upon periodic save

See pad_router.py

@pad_router.delete("/{pad_id}")
async def delete_pad(
    pad_access: Tuple[Pad, UserSession] = Depends(require_pad_owner),
    session: AsyncSession = Depends(get_session)
) -> Dict[str, Any]:
    """Delete a pad (owner only)"""
    try:
        pad, _ = pad_access
        success = await pad.delete(session)
        if not success:
            raise HTTPException(
                status_code=500,
                detail="Failed to delete pad"
            )
        
        return {"success": True, "message": "Pad deleted successfully"}
    except Exception as e:
        raise HTTPException(
            status_code=500,
            detail=f"Failed to delete pad: {str(e)}"
        )

Since the worker can be on another thread/instance of the fastAPI process when running multiple upstreams, we need to find a way to tell the canvas worker that it should stop handling the deleted pad through redis