Indexer: add API key authentication for the GraphQL endpoint

[Haroldwonder]

Summary

indexer/src/graphql.ts serves the GraphQL endpoint with no authentication. Anyone who can reach the indexer can issue arbitrarily expensive queries, and there's no way to attribute usage to a particular consumer for billing or abuse tracking.

Acceptance Criteria

• Optional API-key middleware gating the GraphQL endpoint, configurable via env var.
• Requests without a valid key are rejected with a clear error when auth is enabled.
• Documented in indexer/README.md.

[Ericoluchukwu]

@Ericoluchukwu has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would love to work on this issues because I'm a developer

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[omosvico]

@omosvico has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi can I be assigned to this issue?

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[Onomebello]

@Onomebello has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello i would like to work on this issue

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[drips-wave]

Congratulations, @Onomebello! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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