feat(M7): 第11环裂变引擎端到端打通

- fission_service: 1源任务扇出N套子任务(回填source_fission_id,复用主管道),MAX_FANOUT=5红线
- fission_prompt: 三档参考度(high/mid/low)占位规则,真实措辞待北哥定义
- tasks.py: worker识别裂变子任务→注入对应档位规则进文案上下文(关键缺口修复)
- api/v1/fission: POST触发 + GET进度聚合, main.py注册路由
- 实测: fission_id=2扇出task70/71,worker注入日志level=high,进度端点聚合正确

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -78,6 +78,7 @@ def _register_routers(app: FastAPI) -> None:
     from app.api.v1.delivery import router as delivery_router
     from app.api.v1.stream import router as stream_router
     from app.api.v1.workspaces import router as workspaces_router
+    from app.api.v1.fission import router as fission_router
 
     prefix = "/api/v1"
     app.include_router(auth_router, prefix=prefix)
@@ -90,6 +91,7 @@ def _register_routers(app: FastAPI) -> None:
     app.include_router(delivery_router, prefix=prefix)
     app.include_router(stream_router, prefix=prefix)
     app.include_router(workspaces_router, prefix=prefix)
+    app.include_router(fission_router, prefix=prefix)
 
 
 app = create_app()