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_leak_visual.md

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+# Leak Test: Visual Salience Shortcut (VISUAL Family)
+
+You are detecting whether a question exploits visual salience patterns as a shortcut.
+
+## Question
+
+{question}
+
+## Options
+
+{options}
+
+## Designated Correct Answer
+
+{answer}
+
+## Instructions
+
+Check if the question can be answered by simply picking the most visually salient or "obvious" object/action, without careful visual analysis.
+
+1. Is the correct answer the most prominent or commonly expected element in such a scene?
+2. Are distractor options obviously implausible or uncommon objects/actions for the described setting?
+3. Could a student "game" this question by guessing the most visually dominant element?
+
+If visual salience is an exploitable shortcut → verdict "fail".
+If the question requires careful visual discrimination → verdict "pass".
+
+## Response Format (strict JSON)
+
+```json
+{{"verdict": "pass" or "fail", "reason": "brief explanation"}}
+```