perf(backend): 大纲卷过滤下推 DB WHERE——list_for_project 加 volume 参数

GET /outline?volume=N 原在端点 Python 侧过滤全部章节；现把 volume 条件下推到
OutlineRepo.list_for_project 的 DB WHERE，只查该卷行（少读多余卷 + 语义正确）。
Protocol/SqlOutlineRepo/各 OutlineRepo fake 同步加 keyword-only volume；补 repo 级
下推断言测试。向后兼容：不传 volume 仍返回全部（默认 None）。

apps/api/tests/test_outline.py

@@ -264,6 +264,25 @@ async def test_get_outline_with_volume_filters_to_that_volume() -> None:
     assert all(c["volume"] == 2 for c in vol2_chapters)
 
 
+@pytest.mark.asyncio
+async def test_outline_repo_volume_filter_pushed_into_query() -> None:
+    # 卷过滤下推到 repo（DB WHERE），而非端点 Python 侧过滤：直接调 repo 断言只回该卷。
+    repo = FakeOutlineReadRepo()
+    pid = uuid.uuid4()
+    repo.add_chapter(pid, volume=1, chapter_no=1, beats=["卷一·1"])
+    repo.add_chapter(pid, volume=1, chapter_no=2, beats=["卷一·2"])
+    repo.add_chapter(pid, volume=2, chapter_no=3, beats=["卷二·3"])
+
+    vol1 = await repo.list_for_project(pid, volume=1)
+    vol2 = await repo.list_for_project(pid, volume=2)
+    all_rows = await repo.list_for_project(pid)
+
+    assert [v.chapter_no for v in vol1] == [1, 2]
+    assert all(v.volume == 1 for v in vol1)
+    assert [v.chapter_no for v in vol2] == [3]
+    assert [v.chapter_no for v in all_rows] == [1, 2, 3]
+
+
 @pytest.mark.asyncio
 async def test_get_outline_returns_empty_list_when_no_outline() -> None:
     project_repo = FakeProjectRepo()