Let's cut to the chase
Can employers reject me from a job based on a project like this?
Yes they can. Providing database services for marijuana users is not a protected category, and if employers want to reject you based on that information then can without legal problems.
Whether they will or not is something we can't answer. A substantial number of people are strongly opposed to marijuana and would not want to be associated with it. Some fraction of them might reject you just on the association. Some will suspect that you are an illegal marijuana user, or an addict. Recruiters are generally conservative, in that the problems caused by having an employee who turns out to be a 'pothead' and who gets himself arrested are likely more significant than the benefits of any particular employee.
Against that there may be some that see your app and/or marijuana use in a positive light. It's really your judgement as to whether the former are likely to outnumber the latter in places you will be applying to. In a country with the largest number of people incarcerated for drug offences in the world my bet would be that they, do.
Of course you may decide that the sort of people who would object to your work are not the ones you want to work for. That is entirely up to you.