[Reports] Add receipt resend endpoint for recent activity transactions #709

Goal

Provide the backend API used by the mobile app to resend a receipt email from the recent activity drill-down screen.

Background

The mobile UI now includes a receipt detail page with:

• the selected transaction details
• an email address field
• a resend receipt button

The current mobile implementation only shows mocked UI feedback. This issue covers the backend contract needed to send a receipt email again for a selected transaction.

Requirements

• Add an endpoint that resends a receipt for a specific transaction/receipt reference.
• The endpoint must accept:
  • merchant context / merchant reporting id
  • transaction reference or receipt reference
  • recipient email address
• The endpoint must validate:
  • the email address is present and syntactically valid
  • the reference resolves to an existing transaction for the merchant
• The endpoint should trigger the receipt resend workflow and return a clear success or failure result.
• Prefer a small, explicit request/response contract so the mobile client can provide user feedback without guessing.

Suggested response shape

• success boolean
• message string
• optionally receiptReference / transactionReference if useful for logging or tracing

Acceptance criteria

• A valid request resends the receipt to the supplied email address.
• Invalid email input is rejected with a clear validation error.
• Unknown references return a not-found or equivalent failure response.
• The endpoint is suitable for calling from the receipt detail screen after the user enters an email address and taps resend.
• The endpoint does not require the client to send any data outside the selected transaction and destination email address.

Notes

• This is a separate issue from the search endpoint.
• The mobile app will continue to show local toast feedback, but the actual send action should be handled here.