Machine-readable code for the shared TikTok direct-post capacity rejection

When TikTok's daily active-publisher cap on Zernio's client key is hit, the per-target error arrives as prose ("TikTok direct posting is at capacity right now…") with errorCategory set to user_content or account_issue. Integrators cannot safely branch on prose, and both categories imply something is wrong with the user's content or account, which is not the case. Please add a distinct, stable code on that target, for example quota_exhausted or capacity_shared, and ideally a Retry-After hint. Keeping the internal account-cooldown behaviour is fine. This unblocks automatic retry-then-inbox handling that you yourselves recommend.