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_enumeration.md

@@ -0,0 +1,32 @@
+# Leak Test: Option Length Shortcut (ENUMERATION Family)
+
+You are detecting whether a question exploits option length patterns as a shortcut.
+
+## Question
+
+{question}
+
+## Options
+
+{options}
+
+## Designated Correct Answer
+
+{answer}
+
+## Instructions
+
+Check if the question can be answered by exploiting the length or specificity of options, without actually understanding the video content.
+
+1. Is the correct answer notably longer or more specific than distractors?
+2. Are distractor options clearly shorter, vaguer, or less detailed than the correct answer?
+3. Could a student "game" this question by always picking the longest/most-detailed option?
+
+If option length is an exploitable shortcut → verdict "fail".
+If options are roughly balanced in length and specificity → verdict "pass".
+
+## Response Format (strict JSON)
+
+```json
+{{"verdict": "pass" or "fail", "reason": "brief explanation"}}
+```