Meanwhile, eight years later!
"Invert Mouse" controlling the "up/down" camera angle is *exactly* what I was looking for, and Baloo's response pointed me in the right direction BUT… I assume that the UI has changed since then, because I sitll had to hunt around to find my answer! So, here's an update, with the current path to this solution:
Avatar --> Preferences (Ctrl+P) --> Move & View --> Mouselook --> Mouselook mouse sensitivity --> Invert!
-Sonarr