Multi-currency: corrections, data export, and pro-rata capture

[ItsThompson]

Summary

Implements three multi-currency epic tickets: currency-aware corrections with expense detail, enriched Datarights CSV export with currency snapshots, and pro-rata schedule capture with trusted period context.

What changed

Backend: Expense service

• CorrectExpenseRequest accepts transactionCurrency over REST and gRPC.
• Corrections resolve FX snapshot before any ledger mutation. FX failure leaves the original expense active with no correction row appended.
• Same-currency corrections write an identity snapshot; foreign-currency corrections write a provider snapshot.
• CreateProRataInstallment internal RPC accepts TrustedPeriodContext and CapturedRateSnapshot. Expense validates context consistency, currency catalog membership, and snapshot coverage without calling Finance back.
• fx_client.go adds ConvertWithSnapshot for snapshot-based conversion.
• Proto additions: CorrectExpenseRequest currency fields, CreateProRataInstallment RPC, TrustedPeriodContext, CapturedRateSnapshot.

Backend: Finance service

• CreateProRataExpense requires explicit periodYear and periodMonth. Service clock no longer selects the creation period.
• Validated creation period exists before FX capture or any ledger write.
• FX CaptureRateSnapshot runs before first installment and future schedule inserts. Capture failure returns CONVERSION_UNAVAILABLE with no writes.
• pro_rata_schedules migration adds transaction amount/currency, creation reporting currency, captured snapshot payload, and failure reason fields.
• New FxClient interface and GRPCFxClient for rate capture.

Backend: Datarights service

• expenses.csv gains seven new columns: transaction_amount, transaction_currency, reporting_amount, reporting_currency, exchange_rate, exchange_rate_source, exchange_rate_timestamp.
• budget_periods.csv gains reporting_currency as the fifth column.
• formatMinorUnits formats amounts using each currency minor-unit digits. JPY rows render as plain integers.
• Rows missing required snapshot fields fail the export job.
• export_currency_formatting_fallback_total counter tracks default-settings fallback renders.

Frontend

• Correction form initializes from active expense transaction amount and currency. Submits transactionCurrency.
• Expense detail shows "Period Amount" for same-currency rows, and transaction amount / budget impact / exchange rate / timestamp for foreign-currency rows.
• Correction history renders each row with its own transaction/reporting snapshot and status.
• moneyFacts.ts provides pure helpers for snapshot resolution.

Tests

• services/expense: go build ./... and go test ./... pass. Coverage includes same-currency correction, foreign-currency correction, FX-failure no-write, trusted-context validation, and shared snapshot persistence.
• services/finance: go build ./... and go test ./... pass. Coverage includes missing period fields, FX capture failure, and gRPC handler error mapping.
• services/fx: go build ./... and go test ./... pass.
• services/datarights: go build ./..., go test ./..., and go vet ./... pass. Coverage includes same-currency, foreign-currency, JPY, budget period reporting currency, pro-rata metadata rows, and fallback counter.
• frontend: vitest run passes 54 files, 504 tests. eslint clean. Pro-rata frontend tests pass (10 tests).

Notes

• docker-compose.yml: FX_SERVICE_ADDR=fx-service:9085 added to finance-service for pro-rata capture.
• services/finance/go.mod: added fx module dependency and replace directive.

References

• Ticket 8: Currency-aware corrections and expense detail
• Ticket 12: Enriched data export with currency snapshots
• Ticket 13: Pro-rata schedule capture with trusted period context