feat: add backend BYOK support for GlobalPing integration

[hybrid-zz]

Added backend support for admins to provide their own GlobalPing API key, aligning the implementation closely with the existing Google PageSpeed key integration. When a key is saved, the server automatically attaches an Authorization header to each GlobalPing request, otherwise it will fallback on anonymous checks.

As an additional improvement, I exposed an admin-only settings route that provides status information related to the configured API key:

1. Whether the key is valid or rejected
2. Remaining credits associated with the key
3. Remaining rate limit (per hour, according to the GlobalPing API documentation)

This information could later be used to further enhance the integration. For example:

• Defining a threshold for notifications
• Creating an incident when credits are low or exhausted

My apologies for the previous PR. I misunderstood your request and thought you wanted two separate PRs: one for BYOK and one for Granular Node Control. Don't mind the commits, lol I made a mess.

Partially Fixes #3413

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