Draft changelog: Activity API expiration, OAuth 2.0 webhook signing

[mintlify]

Summary

Adds a Sep 23, 2026 changelog entry for what shipped since the Sep 21 entry.

Changes

• New features: Activity API subscription expires_at, X-Twitter-Webhooks-Signature-OAuth2 webhook signing with the OAuth 2.0 client secret, Livestream API access form
• Updates: is_moderator in broadcast.chat payloads, broadcast scopes in OAuth 1.0a token exchange
• No bug fixes this week

Context

Based on PRs #436–#448. Did not include removing source from the docs (#450), because the OpenAPI spec still lists source in tweet.fields.

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Project	Status	Preview	Updated
x-corp	🟢 Ready	View Preview	Sep 23, 2026, 11:07 PM

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