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----
> [!IMPORTANT]
> Add `STACK_FREESTYLE_API_KEY` to environment variables and update
documentation for email functionality.
>
> - **Environment Variables**:
> - Add `STACK_FREESTYLE_API_KEY` to `docker/server/.env.example`.
> - **Documentation**:
> - Update `self-host.mdx` to require `STACK_FREESTYLE_API_KEY` for
email functionality.
>
> <sup>This description was created by </sup>[<img alt="Ellipsis"
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for d39713a7c5. You can
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **Documentation**
* Updated self-hosting instructions to mention the required
`STACK_FREESTYLE_API_KEY` environment variable for email functionality.
* **Chores**
* Added `STACK_FREESTYLE_API_KEY` to environment configuration files as
a placeholder for the necessary API key.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Co-authored-by: Konsti Wohlwend <n2d4xc@gmail.com>
