Standardise response structure to remove nested items

In the response model of things like Events with many related objects, instead of nesting them like current the response will have different top level objects which are lists of that type of related thing.

For example a event has three related entities: Location, Items, Bundles.

Currently the `EventResponse` model returns like this:

```json
{
"event": {
"status": "draft",
"ksuid": "2vOyCJojl8ozmjx11c1tqwViE5O",
"banner": "https://d232d73chhwq10.cloudfront.net/demo/event/2vOyCJojl8ozmjx11c1tqwViE5O/banner/2914f9ef-5628-46c5-8ae9-6550371ab606.jpg",
"name": "This is my first event",
"starts_at": "2025-04-30T12:34:00",
"ends_at": "2025-04-30T23:45:00",
"category": ["congress"],
"capacity": 120,
"number_sold": 0,
"description": "{string-of-json}",
"created_at": "2025-04-10T08:57:13.595000Z",
"updated_at": "2025-04-10T09:25:06.873000Z",
"version": 8,
"location": {
"ksuid": "2vOzWJBneaaJ6lFokEL18pZGMgF",
"name": "Liverpool Arts Bar",
"address": "22 Hope Street, Hope Street Quarter, Liverpool, Merseyside, L1 9BY",
# "lat": 53.40262,
"lng": -2.96981
},
"items": [...],
"bundles": [...]
}
}
```

the idea is to replace it to return like this:

```json
{
"event": {
"status": "draft",
"ksuid": "2vOyCJojl8ozmjx11c1tqwViE5O",
"banner": "https://d232d73chhwq10.cloudfront.net/demo/event/2vOyCJojl8ozmjx11c1tqwViE5O/banner/2914f9ef-5628-46c5-8ae9-6550371ab606.jpg",
"name": "This is my first event",
"starts_at": "2025-04-30T12:34:00",
"ends_at": "2025-04-30T23:45:00",
"category": ["congress"],
"capacity": 120,
"number_sold": 0,
"description": "{string-of-json}",
"created_at": "2025-04-10T08:57:13.595000Z",
"updated_at": "2025-04-10T09:25:06.873000Z",
"version": 8
},
"locations": [...],
"items": [...],
"bundles": [...]
}
```

**this needs to be implemented across all responses and front end updates at the same time**