feat(question_gen): add lightweight 4-gate quality check

Implement 4 concurrent LLM-based quality gates for generated questions:
- key_verify: validates answer evidence in source material
- blind_answer: rejects questions answerable without video context
- multi_true: detects ambiguous multi-correct options
- leak_test: per-family shortcut detection (5 probe templates)

Includes run_gates orchestrator with verbatim_ratio short-circuit,
JSON response parsing with fallback, and 9 unit tests (all passing).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

store/prompts/question_gen/gate_key_verify.md

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+# Key Verify Gate
+
+You are a quality-control judge for video understanding questions.
+
+## Task
+
+Given the source material from a video and a multiple-choice question with its designated correct answer, determine whether the correct answer is **supported by evidence** in the source material.
+
+## Source Material
+
+{source_text}
+
+## Question
+
+{question}
+
+## Options
+
+{options}
+
+## Designated Correct Answer
+
+{answer}
+
+## Instructions
+
+1. Read the source material carefully.
+2. Determine if the designated correct answer can be derived or inferred from the source material.
+3. If evidence supports the answer, verdict is "pass". If not, verdict is "fail".
+
+## Response Format (strict JSON)
+
+```json
+{{"verdict": "pass" or "fail", "reason": "brief explanation"}}
+```