Replace Event.performer placeholder with real keynote speakers

Problem

The Event schema on the homepage has a generic placeholder for performer:

```json
"performer": {
"@type": "Organization",
"name": "Python Community Speakers"
}
```

Google uses performer to surface speaker rich results. This placeholder adds no value and may be flagged during structured data review.

Fix

Once keynote speakers are confirmed in data/speakers.yml, replace the placeholder with a Hugo range over keynote speakers (the template already exists in structured-data-speakers.html):

```go
"performer": [
{{ range where .Site.Data.speakers "keynote" true }}
{
"@type": "Person",
"@id": "{{ $.Site.BaseURL }}speakers/#{{ .id }}",
"name": "{{ .name }}"
},
{{ end }}
]
```

Until keynote speakers are confirmed, the performer field should be removed from the Event schema entirely rather than using a placeholder.

Files to change

• layouts/partials/structured-data.html

Start in layouts/partials/structured-data.html and compare its Event schema with the existing template in structured-data-speakers.html. Check data/speakers.yml to determine whether keynote speakers are confirmed; the Event schema should either list those speakers or omit performer when none are confirmed.