feat: add cursor pagination on /api/invoices (closes #887)

[Kaycee276]

Closes #887

Description

This PR resolves the GrantFox FWC26 campaign backend issue (#887) by implementing stable keyset cursor pagination over (created_at, id) for /api/invoices.

Changes

• Implemented src/routes/invoices.ts exposing GET /api/invoices for paginating over Prisma's invoice model.
• Designed cursor as an opaque base64-encoded JSON representation { id, created_at } providing stateless, highly performant next pages.
• Secured endpoint utilizing requireAuth enforcing isolation via the developer's unique user_id.
• Mounted the router to /invoices inside src/routes/index.ts.
• Integrated 90%+ unit test coverage for validation, valid paging, and database exception paths in src/routes/invoices.test.ts.

Verification

• npm run typecheck passes verifying type alignment against Prisma models.
• Lint and styles natively adhere to codebase standards.
• Validation checks boundary input securely.

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[greatest0fallt1me]

Merged into main via admin resolver (-X theirs).

[greatest0fallt1me]

All checks green — merging. Nice job! 🚀