Return a stable media id in mediaItems[] that survives temp → permanent promotion

We integrate Zernio for scheduled publishing and delete our own copy of a video

once it is safely with you, so customer disks don't fill up with files that are

already published.

To do that safely we need to confirm that a specific post references the exact

media we uploaded. Today the only thing that crosses from presign to the post

object is the URL string — and that URL is precisely what changes when media is

promoted from temp to permanent storage.

Ana in support confirmed the situation (thanks, that was a very clear answer):

the object name is generated and preserved verbatim, but the path shape is an

internal implementation detail and not a contract, so we shouldn't pin cleanup

to it. There's also no endpoint to resolve a presign key back to its current

publicUrl, and no opaque id in mediaItems[] today.

The request: return a stable media id on each mediaItems[] entry — issued at

presign and echoed unchanged on post create and post read, surviving promotion.

That would let integrators correlate media by identity instead of by string

shape, and it would stay correct if you ever change your storage layout.

A resolve endpoint (presign key → current publicUrl) would also solve it, but an

id is cleaner and cheaper for you to keep stable.

Anyone managing media lifecycle — deleting originals, reconciling after a crash,

auditing what a post actually published — hits this same wall.