#375 Publish /api/posts contract from Marketing (marketing#154)

Publication copy of recoupable/marketing#154, which adds the /api/posts contract (create now or at publish_at, list, get, cancel) to the maintained docs source. Tracking: recoupable/app#2241.

• Synced with Marketing's scripts/sync-legacy-docs.py from branch commit 336bad1; python3 scripts/check_source.py --source <marketing checkout> passes (235 files + navigation).
• Changes: api-reference/openapi/social.json (additive), four pages under api-reference/publishing/, the Connectors → Publishing nav group, and SOURCE-PROVENANCE.json.
• Merge order: marketing#154 first, then re-sync this branch to the squash-merge commit (the pin must point at Marketing main), then merge this.

The contract itself, decisions and checks are described on marketing#154.

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Summary by CodeRabbit

• Documentation
  • Added API reference coverage for creating, listing, retrieving, and cancelling posts.
  • Documented supported platforms, scheduling, filters, pagination, post statuses, and relevant errors.
  • Added a Publishing section to the API documentation navigation.
  • The documented endpoints are marked as pending and unavailable on the hosted API.