feat(question_gen): add generate-v2 CLI subcommand and experiment script

- Add generate-v2 subparser with --config, --store-dir, --db-path,
  --seed, and --dry-run arguments to tools/generate_questions.py
- Implement _run_generate_v2 async handler: config loading, video
  discovery, DI client construction, TreeIndex loading, pipeline
  invocation, and result persistence
- Add scripts/generate_questions_v2.sh following build_trees.sh
  conventions (source .env, conda run python path, MODE=mock support)
- Update app/question_gen/__init__.py to export full v2 public API:
  run_pipeline_v2, PipelineConfig, PipelineResult, QuestionFamilySpec,
  ALL_FAMILIES, CandidateQuestion, generate_one_v2, GateReport, run_gates
- Add QuestionGenStore.load_progress() for pipeline resumption
- Add integration tests for CLI help and dry-run behavior
- Update test_question_gen_api to match expanded __all__

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

app/question_gen/__init__.py

@@ -1,6 +1,10 @@
-"""出题模块 — benchmark 加载、分层采样与赛题合成。"""
+"""出题模块 — benchmark 加载、分层采样、赛题合成与 v2 出题管线。"""
 
+from app.question_gen.families import ALL_FAMILIES, QuestionFamilySpec
+from app.question_gen.gates import GateReport, run_gates
+from app.question_gen.generator_v2 import CandidateQuestion, generate_one_v2
 from app.question_gen.loader import load_benchmark, stratified_sample
+from app.question_gen.pipeline_v2 import PipelineConfig, PipelineResult, run_pipeline_v2
 from app.question_gen.synthesizer import (
     TASK_TYPE_LEVEL_MAP,
     AnchorContext,
@@ -9,10 +13,21 @@ from app.question_gen.synthesizer import (
 )
 
 __all__ = [
+    # v1 接口
     "load_benchmark",
     "stratified_sample",
     "TASK_TYPE_LEVEL_MAP",
     "AnchorContext",
     "generate_one",
     "sample_anchor",
+    # v2 接口
+    "run_pipeline_v2",
+    "PipelineConfig",
+    "PipelineResult",
+    "QuestionFamilySpec",
+    "ALL_FAMILIES",
+    "CandidateQuestion",
+    "generate_one_v2",
+    "GateReport",
+    "run_gates",
 ]