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_multi_true.md

@@ -0,0 +1,28 @@
+# Multi-True Gate
+
+You are checking whether a multiple-choice question has **more than one plausibly correct answer** given the source material.
+
+## Source Material
+
+{source_text}
+
+## Question
+
+{question}
+
+## Options
+
+{options}
+
+## Instructions
+
+1. Read the source material and the question carefully.
+2. For each option, assess whether it could be considered a correct or plausible answer given the source.
+3. If exactly ONE option is clearly correct → verdict "pass".
+4. If TWO or more options are plausibly correct → verdict "fail".
+
+## Response Format (strict JSON)
+
+```json
+{{"verdict": "pass" or "fail", "reason": "brief explanation listing plausible options if multiple"}}
+```