fix: 移除 asset_durations 的 random_preview 条件限制

问题：asset_durations 仅在 random_preview=True 时传入 distribute_assets()，
导致正常创建 plan 时 _calc_random_start_time() 永远走不到随机逻辑，
所有 clip 的 start_time 固定为 0。

修复：移除 random_preview 条件，只要 _asset_repo 存在就始终获取
asset_durations，使素材片段随机起始时间对所有 plan 生效。

新增 3 个测试验证：
- random_preview=False 时也获取 asset_durations
- random_preview=True 时行为不变
- _asset_repo 为 None 时 asset_durations 为 None

apps/api/app/services/plan_generator_service.py

@@ -118,9 +118,9 @@ class PlanGeneratorService:
 
         # 4. 按 editing_mode 分配素材
         if asset_ids:
-            # 如果是随机预览模式，获取素材时长信息
+            # 获取素材时长信息，用于随机起始时间
             asset_durations = None
-            if random_preview and self._asset_repo:
+            if self._asset_repo:
                 asset_durations = self._fetch_asset_durations(asset_ids)
             self._distribute_assets(
                 clips,

tests/unit/test_plan_generator.py

@@ -886,3 +886,134 @@ class TestTemplateConfigPropagation:
         assert clip_cfg.get("speed_ratio") == 1.2
         assert clip_cfg.get("name") == "开场"
         assert clip_cfg.get("custom_field") == "value"
+
+
+class TestAssetDurationsAlwaysFetched:
+    """验证 asset_durations 不再受 random_preview 条件限制。
+
+    修复前：asset_durations 仅在 random_preview=True 时传入 distribute_assets
+    修复后：只要 _asset_repo 存在，就始终获取 asset_durations
+    """
+
+    def _make_service_with_asset_repo(self):
+        """创建带 mock asset_repo 的 PlanGeneratorService"""
+        from apps.api.app.services.plan_generator_service import PlanGeneratorService
+
+        plan_repo = StubEditPlanRepository()
+        clip_repo = StubEditPlanClipRepository()
+
+        # mock asset_repo: 返回带 duration 的素材
+        asset_repo = MagicMock()
+
+        def fake_get(asset_id):
+            mock_asset = MagicMock()
+            mock_asset.duration = 30.0  # 每个素材 30 秒
+            return mock_asset
+
+        asset_repo.get = MagicMock(side_effect=fake_get)
+
+        with (
+            patch(
+                "apps.api.app.services.plan_generator_service.SQLAlchemyEditPlanRepository",
+                return_value=plan_repo,
+            ),
+            patch(
+                "apps.api.app.services.plan_generator_service.SQLAlchemyEditPlanClipRepository",
+                return_value=clip_repo,
+            ),
+        ):
+            db = MagicMock()
+            svc = PlanGeneratorService(db, asset_repo=asset_repo)
+            svc._plan_repo = plan_repo
+            svc._clip_repo = clip_repo
+
+        return svc, asset_repo
+
+    def test_asset_durations_fetched_without_random_preview(self):
+        """random_preview=False 时也应获取 asset_durations"""
+        svc, asset_repo = self._make_service_with_asset_repo()
+
+        template = _make_template("one_take")
+        clip_configs = _make_clip_configs(
+            template_id=template.id,
+            specs=[{"clip_type": ClipType.MAIN, "order": 0, "min_duration": 3.0, "max_duration": 5.0}],
+        )
+
+        # patch distribute_assets 以捕获传入的参数
+        with patch(
+            "apps.api.app.services.plan_generator_service.distribute_assets"
+        ) as mock_distribute:
+            svc.generate_from_template(
+                template=template,
+                clip_configs=clip_configs,
+                asset_ids=["a1", "a2"],
+                random_preview=False,  # 关键：非随机预览模式
+            )
+
+            # 验证 asset_durations 被传入（不是 None）
+            mock_distribute.assert_called_once()
+            call_kwargs = mock_distribute.call_args
+            asset_durations = call_kwargs.kwargs.get(
+                "asset_durations", call_kwargs[1].get("asset_durations")
+            )
+            assert asset_durations is not None, (
+                "asset_durations should be fetched even when random_preview=False"
+            )
+            assert "a1" in asset_durations
+            assert "a2" in asset_durations
+            assert asset_durations["a1"] == 30.0
+
+        # 验证 asset_repo.get 被调用（说明 _fetch_asset_durations 执行了）
+        assert asset_repo.get.call_count >= 2
+
+    def test_asset_durations_fetched_with_random_preview(self):
+        """random_preview=True 时仍正常获取 asset_durations（行为不变）"""
+        svc, asset_repo = self._make_service_with_asset_repo()
+
+        template = _make_template("one_take")
+        clip_configs = _make_clip_configs(
+            template_id=template.id,
+            specs=[{"clip_type": ClipType.MAIN, "order": 0, "min_duration": 3.0, "max_duration": 5.0}],
+        )
+
+        with patch(
+            "apps.api.app.services.plan_generator_service.distribute_assets"
+        ) as mock_distribute:
+            svc.generate_from_template(
+                template=template,
+                clip_configs=clip_configs,
+                asset_ids=["a1"],
+                random_preview=True,
+            )
+
+            call_kwargs = mock_distribute.call_args
+            asset_durations = call_kwargs.kwargs.get(
+                "asset_durations", call_kwargs[1].get("asset_durations")
+            )
+            assert asset_durations is not None
+            assert "a1" in asset_durations
+
+    def test_no_asset_repo_means_no_durations(self):
+        """_asset_repo 为 None 时 asset_durations 应为 None"""
+        svc, _, _ = _make_generator()  # 默认不带 asset_repo
+
+        template = _make_template("one_take")
+        clip_configs = _make_clip_configs(
+            template_id=template.id,
+            specs=[{"clip_type": ClipType.MAIN, "order": 0, "min_duration": 3.0, "max_duration": 5.0}],
+        )
+
+        with patch(
+            "apps.api.app.services.plan_generator_service.distribute_assets"
+        ) as mock_distribute:
+            svc.generate_from_template(
+                template=template,
+                clip_configs=clip_configs,
+                asset_ids=["a1"],
+            )
+
+            call_kwargs = mock_distribute.call_args
+            asset_durations = call_kwargs.kwargs.get(
+                "asset_durations", call_kwargs[1].get("asset_durations")
+            )
+            assert asset_durations is None