feat(backend): AI 服务层 — 千问流式 LLM + 月报解析 + 健康度计算 + 风险检测 + Copilot

- LLM 客户端：全部 SSE 流式输出，兼容 OpenAI 接口
- AI 月报解析：SSE 流式端点 POST /reports/{id}/parse
- 健康度计算引擎：四维评分（财务/经营/AI商业化/AI成本）
- 风险自动检测引擎：6 条规则自动检测指标越界
- AI Copilot：SSE 流式对话 POST /copilot/chat
- 权限中间件：角色级 + 字段级权限控制
- 测试：21 个新测试（健康度 8 + 风险检测 8 + 权限 5），总计 64 passed

backend/app/main.py

@@ -12,6 +12,7 @@ from fastapi.responses import JSONResponse
 
 from app.routers.auth import router as auth_router
 from app.routers.companies import router as companies_router
+from app.routers.copilot import router as copilot_router
 from app.routers.dashboard import router as dashboard_router
 from app.routers.reports import router as reports_router
 from app.routers.risks import router as risks_router
@@ -75,3 +76,4 @@ app.include_router(companies_router, prefix="/api/v1")
 app.include_router(reports_router, prefix="/api/v1")
 app.include_router(dashboard_router, prefix="/api/v1")
 app.include_router(risks_router, prefix="/api/v1")
+app.include_router(copilot_router, prefix="/api/v1")