feat(receipt): register ReceiptModule in AppModule and add AWS SDK deps

[Haroldwonder]

What

The ReceiptModule (service, controller, and tests) was implemented in #509 but never
registered in AppModule, meaning the receipt endpoints were unreachable. This PR completes
the wiring.

Changes

backend/package.json

• Add @aws-sdk/client-s3 and @aws-sdk/s3-request-presigner — direct dependencies of
  ReceiptService (which manages its own S3Client for Cloudflare R2). These were missing from
  package.json and the lock file entirely.

backend/src/app.module.ts

• Import and register ReceiptModule so the following routes are live:
  • GET /v1/transactions/:id/receipt — presigned PDF URL (authenticated owner)
  • POST /v1/transactions/:id/receipt/email — email receipt to user
  • GET /v1/paylinks/:tokenId/receipt — presigned PDF URL for paid PayLink
  • GET /v1/admin/transactions/:id/receipt — already wired via AdminModule ✓

What was already in place (from #509)

File	Description
receipt.service.ts	generateTransactionReceipt, generatePayLinkReceipt,
emailTransactionReceipt, generateTransactionReceiptAdmin with R2 HeadObject cache check
receipt.controller.ts	All three user-facing endpoints with JWT guard and ?refresh=true
support
receipt.module.ts	Module wiring (TypeORM repos, r2Config, EmailModule)
receipt.service.spec.ts	Unit tests: own tx receipt, 403 for other user, cached receipt
not regenerated, force refresh, PayLink creator/payer, email queuing
admin.controller.ts	GET /admin/transactions/:id/receipt endpoint

Testing

bash

Unit tests (all passing)

cd backend && pnpm test src/receipt/receipt.service.spec.ts

closes #525