docs: document proposal override flow in specs

[Scr4tch587]

Documents the stale-branch conflict/override flow added across this stack in both specs.

What changed:

• `SPEC-backend.md`: new "Conflict handling" subsection under the dataset proposals endpoint explaining the `dataset_exists` vs `stale_branch` `conflict_type`, `open_pr_url`, and how `override` resolves the recoverable case; the 409 status table row now points there instead of describing both cases inline
• `SPEC-frontend.md`: new "Stale-proposal conflicts" paragraph under the create-dataset form section describing the confirm dialog and resubmit-with-override behavior

No code changes.

[github-actions]

• main
  • feat: add branch/pr cleanup methods to GitHubClient #394
    • feat: distinguish dataset-exists vs stale-branch conflicts in proposals #395
      • feat: accept override flag on POST /datasets, shape conflict responses #396
        • feat: confirm-and-override dialog for stale proposal conflicts #397
          • docs: document proposal override flow in specs 👈

[eu-lee]

Proposal looks accurate