feat: allow dynamic prevent-going-back conditions

[jpagh]

This PR makes prevent going back work like back button or other interview keys that evaluate a Python expression for truthiness. It works on one line, or with a scalar.

prevent going back: user_has_signed or payment_processed

or

prevent going back: |
  user_has_signed or payment_processed

In my case, I want this added because I want to be able to do this for development so that I can go back, but my users in production can't:

prevent going back: not user_has_privilege(["developer", "admin"])

[Copilot]

Pull request overview

This PR extends the prevent going back directive so it can be driven by a dynamic Python expression (similar to back button), allowing the “Back” navigation to be conditionally enabled/disabled per question render.

Changes:

• Parse prevent going back as either a boolean or an evaluatable Python expression and compute can_go_back at ask-time.
• Plumb can_go_back through the question result payload into InterviewStatus.
• Update HTML rendering to respect status.can_go_back (evaluated) instead of question.can_go_back (could be a code object).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docassemble_base/docassemble/base/standardformatter.py	Uses evaluated status.can_go_back to decide whether to render the back button.
docassemble_base/docassemble/base/parse.py	Computes/evaluates dynamic “prevent going back” conditions and propagates the result into InterviewStatus.

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