Add invoice payment velocity tracker

[Kingsman-99]

Label: complexity: high
Points: 200

Description

There is no SDK method to analyse how quickly payments are arriving for an invoice. This issue adds trackVelocity(address) that computes payment rate analytics across all invoices for an address, identifying fast-filling and stalled invoices.

Technical Context

Involves a new src/velocityTracker.ts. Fetch all invoices for address. For each, compute paymentsPerDay from payment timestamps. Define VelocityReport = { address: string; invoices: Array<{ invoiceId: string; paymentsPerDay: number; trend: 'accelerating' | 'steady' | 'stalling' }> }. Trend based on comparing first-half vs second-half payment rate.

Acceptance Criteria

• trackVelocity(address: string): Promise<VelocityReport> exported
• paymentsPerDay computed from actual payment timestamps
• trend correctly classified for accelerating, steady, and stalling patterns
• VelocityReport type exported from src/index.ts
• Test verifies 'stalling' trend for invoice with decreasing payment rate
• All existing tests pass
• TypeScript strict mode — zero any types

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[OSEH-svg]

@OSEH-svg has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello Maintainer,
I’d love to work on this issue. I'm a software developer with 5 years of experience and I’m confident I can deliver a good solution. Please feel free to assign it to me.

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[Oluwasuyi-Timilehin]

@Oluwasuyi-Timilehin has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello maintainers I'm a full stack developer experience in building scalable systems I can handle this perfectly if assigned

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[Silver36-ship-it]

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I would love to work on this professionally, please assign it to me

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[GauravKarakoti]

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Allow me to work on this one

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[therealjhay]

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Hello Maintainer, i would love to work on this

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[Mmesolove]

@Mmesolove has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer, I have read the description and I can solve it,please kindly assign to me

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[veemakama]

@veemakama has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

i would like to work on this

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[wendypetersondev]

@wendypetersondev has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer i would like to work on this issue!!

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[drips-wave]

Congratulations, @wendypetersondev! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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