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/fakes_projects.py

@@ -358,8 +358,14 @@ class FakeOutlineReadRepo:
     async def get(self, project_id: uuid.UUID, chapter_no: int) -> OutlineView | None:
         return self.rows.get((project_id, chapter_no))
 
-    async def list_for_project(self, project_id: uuid.UUID) -> list[OutlineView]:
-        views = [v for (p, _), v in self.rows.items() if p == project_id]
+    async def list_for_project(
+        self, project_id: uuid.UUID, *, volume: int | None = None
+    ) -> list[OutlineView]:
+        views = [
+            v
+            for (p, _), v in self.rows.items()
+            if p == project_id and (volume is None or v.volume == volume)
+        ]
         return sorted(views, key=lambda v: v.chapter_no)