Build invoice notification templates

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Apps sending invoice event notifications must format messages manually. This issue adds a NotificationTemplate system with built-in templates for each invoice event type and support for custom template strings with variable interpolation.

Technical Context

Involves a new src/notificationTemplates.ts. Define built-in templates for created, payment, released, refunded, expiring events. renderTemplate(event: InvoiceEvent, template?: string): string interpolates {{invoiceId}}, {{amount}}, {{creator}} variables. Custom templates override built-ins.

Acceptance Criteria

• renderTemplate(event, template?): string exported from src/index.ts
• Built-in templates for all 5 event types
• Custom template string overrides built-in when provided
• {{invoiceId}}, {{amount}}, {{creator}} variables interpolated
• Test verifies rendered output for each built-in template
• All existing tests pass
• TypeScript strict mode — zero any types

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[khapyinyass]

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

Hi maintainer
I'd love to work on this repo

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[Eromosele0110]

@Eromosele0110 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I will like to solve this issue

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[drips-wave]

Congratulations, @Eromosele0110! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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