docs(backend): SSE 端点在 OpenAPI 如实声明 text/event-stream

draft/rewrite/review 三个流式端点返回 StreamingResponse，OpenAPI 原按默认
application/json 标注 200，schema 不诚实。新增共享 _SSE_RESPONSE 声明 200 的
content 为 text/event-stream 并挂到三个装饰器。export_openapi 通过，schema 现
出现 3 处 text/event-stream。未跑前端 gen:api（按要求）。

apps/api/ww_api/routers/projects.py

@@ -105,6 +105,13 @@ _ACCEPT_ERRORS: dict[int | str, dict[str, Any]] = {
     409: {"model": ErrorEnvelope, "description": "存在未裁决冲突"},
     503: {"model": ErrorEnvelope, "description": "LLM 不可用"},
 }
+# SSE 端点：如实声明 200 的 media type 为 text/event-stream（否则 OpenAPI 误标 json）。
+_SSE_RESPONSE: dict[int | str, dict[str, Any]] = {
+    200: {
+        "description": "SSE 事件流（`event: <name>\\ndata: <json>`，见 memory/contracts.md）",
+        "content": {"text/event-stream": {}},
+    }
+}
 
 ProjectRepoDep = Annotated[ProjectRepo, Depends(get_project_repo)]
 ChapterRepoDep = Annotated[ChapterRepo, Depends(get_chapter_repo)]
@@ -256,7 +263,7 @@ def _encode_sse(event: SseEvent) -> str:
     return f"event: {event.event}\ndata: {payload}\n\n"
 
 
-@router.post("/{project_id}/chapters/{chapter_no}/draft")
+@router.post("/{project_id}/chapters/{chapter_no}/draft", responses=_SSE_RESPONSE)
 async def stream_draft(
     project_id: uuid.UUID,
     chapter_no: int,
@@ -319,7 +326,7 @@ async def stream_draft(
     )
 
 
-@router.post("/{project_id}/chapters/{chapter_no}/rewrite")
+@router.post("/{project_id}/chapters/{chapter_no}/rewrite", responses=_SSE_RESPONSE)
 async def stream_rewrite(
     project_id: uuid.UUID,
     chapter_no: int,
@@ -532,7 +539,7 @@ async def _resolve_review_draft(
     return saved.content
 
 
-@router.post("/{project_id}/chapters/{chapter_no}/review")
+@router.post("/{project_id}/chapters/{chapter_no}/review", responses=_SSE_RESPONSE)
 async def review_chapter(
     project_id: uuid.UUID,
     chapter_no: int,