feat(analytics): add revenue-by-currency report

[retkatmun]

Closes #1757

Summary

Multi-currency revenue is now grouped per currency with monthly/annual totals, subscription counts, averages, and percentage shares. Reports are exportable as CSV or JSON via a new HTTP endpoint.

Changes

New files:

File	Purpose
backend/services/analytics/revenueByCurrencyReport.ts	Core service: generateReport() groups subscriptions by currency, computing per-currency MRR, ARR, average, and percentage. toMonthlyEquivalent() normalises billing cycles (monthly/yearly/weekly/custom). exportReport() produces CSV (with totals row) or pretty-printed JSON.
backend/services/analytics/revenueByCurrencyRouter.ts	Plain Node.js HTTP router. `GET /reports/revenue-by-currency?merchantId=…&format=csv
backend/services/analytics/__tests__/revenueByCurrencyReport.test.ts	27 unit tests covering all paths.

Modified:

• backend/server.ts — wires the service and router; route placed before the 404 fallback.

Design decisions

• Decoupled from data layer. The service accepts SubscriptionRevenueInput[] — a minimal shape with amount, currency, billingCycle, and optional status. This maps to both the frontend Subscription.price and backend Subscription.amount without coupling to either.
• Billing cycle normalisation. toMonthlyEquivalent() converts yearly (÷12), weekly (×52/12), and custom/unknown (as-is) to a monthly equivalent so all currencies are comparable.
• CSV with totals row. The CSV export includes a header row, one row per currency, then a totals row — suitable for spreadsheet import.
• Currency normalisation. Currency codes are uppercased and grouped (e.g. "usd" and "USD" are the same group).
• Status filtering. Only active subscriptions (or those with no status) are included; paused and cancelled are excluded.

Acceptance criteria (from issue)

• Per-currency totals — each currency gets MRR, ARR, count, average, and percentage
• Exportable — CSV and JSON formats via ?format= query parameter
• Tested — 27 unit tests, all passing

Testing

npx jest --config jest.backend.config.js \
  --testMatch '**/backend/services/analytics/__tests__/revenueByCurrencyReport.test.ts'

Test Suites: 1 passed
Tests:       27 passed, 0 failed

Covers: toMonthlyEquivalent() for all billing cycles, generateReport() (empty input, status filtering, single/multi-currency, percentage calculation, sorting, currency normalisation, billing cycle normalisation), exportReport() JSON round-trip, CSV headers/rows/totals/escaping.

[drips-wave]

@retkatmun Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits