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_blind_answer.md

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+# Blind Answer Gate
+
+You are evaluating whether a multiple-choice question can be answered correctly **without any video context**, using only common sense or option patterns.
+
+## Question
+
+{question}
+
+## Options
+
+{options}
+
+## Instructions
+
+1. Read the question and options WITHOUT any source material.
+2. Try to determine the correct answer using only common sense, option length patterns, or grammatical cues.
+3. If you can confidently pick the correct answer → verdict "fail" (the question leaks information).
+4. If you cannot determine the answer without context → verdict "pass" (the question genuinely requires video understanding).
+
+## Response Format (strict JSON)
+
+```json
+{{"verdict": "pass" or "fail", "reason": "brief explanation"}}
+```