Pool pump shaft seal leak...or something else?
I replaced the motor, and that seems to have fixed the leak.

I suspect that during my first attempt to replace the shaft seal, I damaged something in the motor while struggling to remove the impeller.  After that, the shaft was probably slightly off center, such that the two sides of the seal were not properly aligned.

It's possible that just replacing the bearings might have fixed the problem too, but I didn't feel confident that I could do that successfully.  And the typical cost to pay someone else for that is about the same as a new motor anyway.