Task 008: Boundary Questions Loop

Development description: Implement generation, editing, partial saving, and submission of boundary questions before plan generation.

Implementation Details

Backend endpoints:
- POST /api/articles/{article_id}/boundary-questions/generate
- GET /api/articles/{article_id}/boundary-questions
- PATCH /api/articles/{article_id}/boundary-questions/{question_id}
- POST /api/articles/{article_id}/boundary-questions/submit
Generate 5-10 questions covering:
- Audience.
- Purpose.
- Reader outcome.
- Depth.
- Tone.
- Excluded topics.
- Primary keyword.
- Competitor angle.
- Evidence standard.
- Visual expectations.
Use agent job path with fake runner fixtures for tests.
Store editable questions and answers.
Block plan generation until all required questions are answered and submitted.
Frontend:
- Boundary questions screen.
- Partial save.
- Required answer validation.

TDD pre-requirement: before implementation, write one failing API behavior test for required unanswered questions blocking submission; proceed through generate/save/submit behavior one cycle at a time and record evidence in Result.
Questions are generated from article brief plus target site config.
Required categories are represented.
Editor can save partial answers.
Required unanswered questions block submission.
Submission writes workflow event and updates article status.
Plan generation remains blocked until boundary answers are submitted.
Frontend displays required/optional state clearly.

Run boundary question API tests.
Run frontend page tests.
Run a Docker Compose smoke flow from article creation to boundary answer submission.