Graduate a trial reel via the API

Today trialParams.graduationStrategy can only be set at creation. With MANUAL, the actual "share to followers" step has to be done by hand in the Instagram app, which breaks the workflow for anyone publishing through the API: the reel is created programmatically, but the decision that makes it reach the audience is not.

Two things would close the loop:

1. An endpoint to graduate an existing trial reel (share it to followers) by post ID, so the decision can be taken from the tool that published it.

2. A way to know that a graduation happened — a field on the post or a webhook — including for SS_PERFORMANCE. Right now, when Instagram auto-graduates, we never learn about it, so we cannot tell whether a reel reached our followers, and any performance comparison between a trial reel and a normal post is unreliable.

Of the two, (2) matters more: without it we're publishing into a state we can never read back.