feat: 商机跟单默认筛选跟进中 + 已流失置底 + 清空列收

- 后端: 状态改为已流失时自动清空 expected_revenue_date
- PC端: 默认筛选跟进中，排序 跟进中→已签约→已流失
- 移动端: 列表自动排序，已流失在最底部

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

backend/app/api/mini_business.py

@@ -104,6 +104,9 @@ async def update_mini_business(
         update_data.pop("manager_id", None)
     for k, v in update_data.items():
         setattr(m, k, v)
+    # Auto-clear expected revenue date when status becomes 已流失
+    if m.status == "已流失" and m.expected_revenue_date:
+        m.expected_revenue_date = ""
     new_snapshot = {"customer_id": str(m.customer_id), "product_type": m.product_type, "amount": m.amount, "follow_up_detail": m.follow_up_detail, "status": m.status, "expected_revenue_date": m.expected_revenue_date}
     changes = compute_diff(old_snapshot, new_snapshot)
     if changes: