fix(ci): keep Discord invite check green on fork PRs - #1717

This PR only touches the Discord-invite job (removing the job-level if that made the check report a failure on fork PRs, and moving the "skip posting" decision into the script), plus its workflow. It does not touch any Python source.

Two checks are red on the head commit, and I don't believe either is caused by this change:

• test (windows-latest) — the End-to-end Tests workflow is already failing on main. The last two runs on main (09/06 15:46 and 09/06 15:31) both concluded failure, so this looks pre-existing rather than introduced here.
• build (3.13) — fails at the Run Unit and Integration Tests step, while build (3.12) and build (3.14) in the same matrix pass, and Build Test on main is green across the last several days. That pattern points at a flaky or environment-specific test rather than a regression from this change.

For visibility, post_discord_invite on this head commit now reports success — which is the behaviour the PR is meant to restore.

Could someone with write access re-run the two failed jobs? If they come back green, this should be good to go. Happy to dig into either one if it turns out to be real.