Photos Haz Repair
We all hope to never need it, but if you do, there's a built-in repair routine in Photos for OS X. Just as in Aperture, hold down Command and Option on launch and you'll be presented with this dialog…
Notice there's no “Repair Permissions, Repair Database, or Rebuild Database” options like we had in Aperture. It's just a one-off fix.
Hopefully you won't need it, but if you do, there it is.