Raw pointer return from RedisConnection::subscriber() risks memory leak

## Description

In `RedisConnection.hpp:558-567`, `subscriber()` returns a raw owning pointer:

```cpp
swr::Subscriber* subscriber()  // caller must delete
```

If the caller forgets to delete or an exception occurs between allocation and deletion, this leaks memory.

## Severity
Medium

## Suggested Fix
Return `std::unique_ptr<swr::Subscriber>` instead of a raw pointer.