Cancel source extraction deadlines and isolate scheduler failures

[AyobamiH]

Source extraction could time out while its paid AI request continued running. Separately, a scheduling exception could prevent later tenants and publishing stages from being processed.

This PR fixes both failure paths and targets OneClickPostFactory/social-agents → main.

Changes:

• Propagate extraction cancellation through AI requests, HTTP response reads and retry backoff.
• Await request settlement before releasing the source claim.
• Isolate scheduler failures by stage, tenant and due queue row, allowing unrelated work to continue.
• Expose scheduler error counts.
• Add regression tests for cancellation and scheduler failure isolation.
• Use Node’s tsx import hook for builds to avoid the CLI’s IPC listener requirement.
• Update the implementation checkpoint with completed work and remaining gaps.

Validation recorded for commit c657892a429ad0e191eb9d42e04f4b3c427041bc:

• Local npm run ci passed.
• [Fork PR build and tests passed](https://github.com/AyobamiH/social-agents/actions/runs/34724185936).
• [Branch build and tests passed](https://github.com/AyobamiH/social-agents/actions/runs/34724099778).
• Tests intercepted external transport; no paid API calls or live social posts were made.

The repair commit builds directly on upstream e9eb7fa8b2d10146480377e8c51854991c728194, preserving the existing work.

This is the intended upstream integration of the patch reviewed in [fork PR #6](AyobamiH#6). That fork PR remains a draft review copy and should not be merged.

Companion application changes: [App PR #9](https://github.com/AyobamiH/oneclickpostfactory/pull/9).

This PR contains no database migrations or production deployment. The broader reliability work remains tracked in docs/implementation-checkpoint-20260912.md.