storageUsageEstimator.ts: Add breakdown by data category in usage report

[Kingsman-99]

Description

src/storageUsageEstimator.ts reports total estimated storage usage as a single number. Integrators need a breakdown by category (invoices, events, cache) to identify which data type is driving storage costs.

Acceptance Criteria

• estimateUsage() returns a StorageReport with a breakdown: Record<string, number> field
• breakdown keys correspond to data categories registered with the estimator
• breakdown values sum to the existing totalBytes field
• Unit tests pass

Context

• Target file: src/storageUsageEstimator.ts
• Categories should be registered via a registerCategory(name, estimateFn) method

[veloura-dev]

@veloura-dev has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi MAINTAINER, I'd like to take this on.
I understand what's being asked here and I'm confident I can implement this correctly and efficiently.

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[Devcyprian]

@Devcyprian has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

can i work on this ,trust me on this

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Congratulations, @Devcyprian! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@Devcyprian's PR #719 was approved and merged by @Kingsman-99.

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👏 Great work, @Devcyprian! Keep contributing to Stellar-split.

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This issue has been completed by @Devcyprian as part of the Stellar Wave Program's 8th Wave 🥳

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