Document expires_at on X Activity API subscriptions

[mintlify]

Adds a "Subscription expiration" section to the X Activity API introduction covering the optional expires_at field on subscription creation, when subscriptions persist without it, and how re-creation refreshes the value.

The POST, GET, and PUT route pages render directly from openapi.json, which already includes expires_at on the create request and on the create/get/update subscription response objects — so those pages pick up the field automatically with no edits needed.

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Requested by: mrosinsky@x.ai via Slack
Mintlify session: slack_1790098106.507119_C09MQDT6BM4

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[mintlify]

Docs PR opened: #442

Added a Sep 22, 2026 changelog entry announcing the optional expires_at field on X Activity API subscriptions.