Draft changelog: streaming latency and webhook auth updates

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Summary

Adds two changelog entries covering product changes shipped since the Sep 1 entry.

Changes

• Sep 4: expanded authentication options — OAuth 2.0 user context now accepted on /2/webhooks management endpoints, and GET /2/webhooks plus X Activity subscription update/delete endpoints now accept user-context auth in addition to app-only Bearer Tokens.
• Sep 2: streaming P99 latency improved from ~6-7 seconds to ~4-5 seconds on hydration-prioritizing streams (Filtered Stream, volume streams).

Context

Sourced from changes merged to main since the last changelog entry: the streaming latency update and the OpenAPI spec syncs reflecting new accepted auth methods. Docs-only changes were excluded.

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