fix(github): improve GitHub App setup and installation flow

- resolve the GitHub App by a stable identifier during installation
  callbacks so installing and re-installing keeps working over the
  full lifetime of the App
- verify the installation id received from the callback against the
  GitHub API before persisting it
- support re-installing an already configured GitHub App instead of
  blocking it
- require an authenticated session and rate limit the setup callback
  routes
- extend manifest setup state validity to match GitHub's manifest
  code lifetime

Adds feature coverage for the GitHub App setup and installation
callbacks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>