Fill gaps from assistant conversations: Free-plan API 401 restricted key

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Summary

Document the restricted-key 401 Unauthorized that Free-plan accounts hit when calling any endpoint other than POST /v2/tracking_requests, so readers can self-diagnose without asking the assistant.

Changes

• Added a "Restricted API access on Free plans" subsection to api-docs/api-reference/introduction.mdx covering the exact error message, which endpoint still works, and how to enable full API access (support-enabled 7-day trial or a paid plan).
• Added a row to the entitlements table in api-docs/useful-info/entitlements.mdx for full API read access, mirroring the same error string and remediation path.

Context

Assistant conversations in the review window repeatedly answered variants of "401" and "You do not have permissions for using the API, except for creating tracking requests" with support-only guidance. These docs surface the answer directly.

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