Add filtering conversations by open/closed to the project conversation list

# Problem

The conversation list for projects needs to have an `open/closed` filter placed on it.

We should show a (styled) `select` that has two items:

- Open (3)
- Closed (18)

Where the number in parens is the number of currently open or closed conversations.

This is blocked by implementing a counter cache for open and closed conversations on the API.

没有指定文件、测试或入口点。首先定位项目的会话列表，并检查用于统计打开和关闭会话数量的 API 缓存工作。完成标准是：一个带样式的 select 显示“打开”和“关闭”及当前数量，并相应地筛选列表。

由索引模型根据 Issue 内容生成。