fix(question_gen): L3 frame_path 拼接视频目录前缀

tree.json 中 frame_path 是相对路径（frames/xxx.jpg），
generate_questions 需要拼上 store/videos/{video_id}/ 前缀供 VLM 读取。

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

tools/generate_questions.py

@@ -918,6 +918,14 @@ async def _run_generate(args: argparse.Namespace) -> None:
                 total_failed += 1
                 continue
 
+            # frame_path 是相对于视频目录的，拼为绝对路径供 VLM 读取
+            video_dir = videos_dir / video_id
+            for l1 in tree.roots:
+                for l2 in l1.children:
+                    for l3 in l2.children:
+                        if l3.frame_path and not Path(l3.frame_path).is_absolute():
+                            l3.frame_path = str(video_dir / l3.frame_path)
+
             used_node_ids: set[str] = set()
             session_id = f"gen-{task_type}-{seq}"
             generated = False