Indexer: add GraphQL query depth/complexity limiting

[Haroldwonder]

Summary

indexer/src/schema.graphql allows deeply nested queries (e.g. attestations → issuer → attestations) with no depth or complexity limit, making the endpoint vulnerable to a single malicious query exhausting database connections.

Acceptance Criteria

• A query complexity/depth limiter is added to the GraphQL server setup in indexer/src/graphql.ts.
• Exceeding the limit returns a GraphQL error rather than executing the query.
• A test confirms an over-limit query is rejected.

[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

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Onomebello to this issue.

[drips-wave]

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

🧑‍💻 @Onomebello: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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