Draft changelog: user-context auth and faster streaming

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Summary

Adds a Sep 4, 2026 changelog entry covering product changes shipped since the last entry (Sep 1).

Changes

• New authentication options: webhook management endpoints and X Activity API subscription update/delete endpoints now accept OAuth 2.0 user-context (and OAuth 1.0a where noted) in addition to app-only Bearer Tokens.
• Streaming latency improvement: P99 latency on hydrated streams (Filtered Stream, volume streams) improved from ~6-7 seconds to ~4-5 seconds.

Context

Sourced from merges to main since the last changelog entry: the OpenAPI spec sync adding user-context auth to webhook/Activity endpoints, and the streaming P99 latency update.

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Project	Status	Preview	Updated
x-corp	🟢 Ready	View Preview	Sep 4, 2026, 5:11 PM

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