May use the San Francisco font for my web apps?
This question generally answers the question of legality.

Including these font files in your app and using them with @font-face or @import is unquestionably not permitted.

However, for a web app you can non-explicitly use it by specifying special keywords in your CSS:

body {
  font-family: system-ui, -apple-system, BlinkMacSystemFont, "Helvetica Neue",
      "Lucida Grande", "Segoe UI";
}


This will, of course, result in slightly different appearances per platform and version (namely non-Apple platforms and Apple systems before iOS 9/OS X 10.11).