Draft changelog: Sep 9 entry for streaming and auth updates

[mintlify]

Summary

Adds a Sep 9, 2026 changelog entry covering product changes shipped since the Sep 1 entry.

Changes

• Streaming P99 latency improved from ~6-7 seconds to ~4-5 seconds on volume and filtered stream endpoints.
• broadcast.read and broadcast.write OAuth 2.0 scopes added to the scopes table (PR Add broadcast scopes to OAuth 2.0 scopes table #350).
• Webhook management and X Activity subscription endpoints now accept OAuth 2.0 user context (and OAuth 1.0a where newly supported) per the OpenAPI spec.

Context

Reviewed all commits and merged PRs on main since the last changelog entry (Sep 1); remaining OpenAPI spec churn was scope-array reordering with no user impact.

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Project	Status	Preview	Updated
x-corp	🟢 Ready	View Preview	Sep 9, 2026, 8:25 PM

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