`update_event()` returns cached events list instead of the API update response

[Olen]

Caught during the pdoc/docstring audit in #237.

`Spond.update_event()` is annotated `-> list[JSONDict] | None` and the docstring says it returns "json results of post command". But the implementation at `spond/spond.py:451-455`:

```python
async with self.clientsession.post(
url, json=base_event, headers=self.auth_headers
) as r:
self.events_update = await r.json()
return self.events
```

The API response is saved to `self.events_update` but the method returns `self.events` — the unrelated cached events list (potentially `None` if `get_events()` was never called). So callers asking "did my update succeed, and what did the server return?" get the wrong thing back.

Looks like a typo from a long-ago refactor: assigning the result to `self.events_update` reads like an intentional "don't trample the cache", but then returning `self.events` instead of `self.events_update` (or just `await r.json()`) is almost certainly unintended.

Suggested fix

Pick one of:

```python

Option A: just return the response directly, drop the side-effect attribute

return await r.json()

Option B: keep the attribute, return it

self.events_update = await r.json()
return self.events_update
```

Option A is cleaner — `self.events_update` isn't referenced anywhere else in the codebase, so keeping it just creates an extra attribute on the class.

Notes

• No tests currently cover `update_event()`, which is why this hasn't been caught.
• The docstring's intent ("json results of post command") is correct — the bug is purely in the code.
• The bug was also reproduced earlier today during research for PR fix: fetch single event by uid directly (#137, #138) #236: `update_event` calls `_get_entity(EVENT, ...)`, which fix: fetch single event by uid directly (#137, #138) #236 makes correct for events outside the 100-event window. fix: fetch single event by uid directly (#137, #138) #236 incidentally fixes the lookup half of `update_event`, but this return-value bug is independent and remains.