collab: Use a separate Anthropic API key for Zed staff (#16128)

This PR makes it so Zed staff can use a separate Anthropic API key for
the LLM service.

We also added an `is_staff` column to the `usages` table so that we can
exclude staff usage from the "active users" metrics that influence the
rate limits.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>