feat: add grounded distractor selector with visual scoring

store/prompts/question_gen/ar_distractor_pool.md

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+You generate hard-negative distractor options for a video Action Recognition
+multiple-choice question.
+
+## Given
+- The question, the correct answer, subtitle context, and video frames.
+
+## Rules
+- Produce distractors that are **grounded near-misses**: each MUST describe an
+  action/entity that genuinely appears in the video, differing from the correct
+  answer in exactly ONE dimension (timing, subject, manner, or object).
+- NEVER invent events absent from the video ("negative space"). A distractor
+  that names something not shown is a failure.
+- Each distractor must be a plausible answer to the question for someone who
+  only skimmed the video.
+- Keep each distractor parallel in structure and length to the correct answer.
+
+## Output
+Respond with ONLY a JSON object:
+```json
+{"distractors": ["...", "...", "..."]}
+```
+Return exactly N distractors (N is given in the request). No option-letter
+prefixes, just the raw text.