Post platform deleted event

Webhook payload for the per-platform terminal events post.platform.published and post.platform.failed, for post.platform.deleted (same shape, fired when Zernio's background sync detects that a platform target published through Zernio was later deleted on the platform; poll-driven ~hourly, not real-time), and for post.tiktok.url_resolved (same shape, fired when a published TikTok post's public URL is backfilled). Terminal events fire once per platform target inside a post as that platform reaches a terminal state (published or permanent failure). The post envelope mirrors the shape of WebhookPayloadPost so consumers can reuse rendering logic; the platform block identifies which specific platform transitioned; the account block identifies the connected social account behind that platform-write.