Allow minting a replacement short link for small shared plans

[backnotprop]

Context

PR #1425 fixes #798 by treating incoming short links as immutable snapshots and removing a stale short URL after the represented content changes.

A remaining product limitation affects small markdown plans: ExportModal only offers Create short link when the full hash URL exceeds 2048 characters or hash sharing is unavailable. After editing a small plan loaded from a short link, the user gets a correct fresh full URL, but cannot explicitly mint a replacement short link from Export. This leaves the literal short-link step in the workflow reported in #798 incomplete for small plans.

Intended behavior

After an incoming or locally generated short URL is invalidated by a material edit, Export should offer an explicit way to create a fresh short link even when the current full hash URL is below the large-URL threshold.

Constraints

• Keep pastes immutable: never update or delete the original paste.
• Creating a replacement remains an explicit user action; do not auto-upload on edits.
• Preserve current payload, encryption, and existing-link compatibility.
• Decide whether the action should appear only after invalidation or be available for every shareable small plan.

Acceptance criteria

• A small markdown plan loaded from a short link loses the stale URL after an edit.
• Export exposes a replacement short-link action.
• Invoking it creates a distinct encrypted paste representing current content.
• The original paste remains unchanged.

Tracked separately from #1425 per the maintainer review so that PR stays focused on stale-snapshot correctness.