Very pretty girl!
I agree with the others, she seems cowhocked which is very common in BCs and shouldn't be an issue at all. I don't love her topline in the first picture. Mostly though, I feel she could use better muscling but that could just be age related if she's young. The top photo really looks like a fairly young dog to me and if so, she can change a lot as she grows and matures. From what can be seen in the bottom photo, I think she looks better there all around. Do you have a picture of her standing like that from the side? On her own and alert?
I don't really agree that "agility is hard on all dogs" though. Agility should be pretty easy for a fit dog with relatively sound structure. What my dogs do when they are loose in the woods is far "harder" than anything they do when we practice or run agility. My GSD had an east-west front, a missing toe and was roach backed and she competed until she was about 11. And she was a crazy dog, never careful about anything in her life. She did develop some arthritis in old age but nothing abnormal for her breed (actually, I've known plenty of couch potato GSDs who have been far worse off in that area).
So I wouldn't worry about if her structure is going to affect her in agility. She seems to have more than adequate structure for that. I would just be sure she is very fit and well conditioned. And by that I mean she has good muscle, no extra weight and you work on tricks which encourage flexibility, coordination and balance on a very regular basis. That stuff is far more important than worrying about structural imperfections that you can't do much about anyway.