Multi-currency: pro-rata future application and legacy failures

[ItsThompson]

Summary

Reworked the Finance pro-rata future-application flow so pending installments are applied only after their target budget period exists. Reporting amounts derive from the captured schedule snapshot. No live provider rates are called. Legacy schedules without captured snapshots are handled without invented rates. Deterministic failures move a schedule to failed with a typed reason. Transient failures leave the schedule pending for retry. Finance marks a row applied only after the Expense ledger write succeeds.

What changed

Backend: Finance service

• Rewrote applyPendingProRata to accept the target period, resolve reporting currency locally, and pass trusted period context plus captured snapshot to Expense. Each schedule is applied independently so one failure does not block the rest.
• Added applyOneProRataSchedule (captured-snapshot path) and applyLegacyProRataSchedule (legacy migration path).
• Added MarkProRataFailed repository method and SQL query to move schedules to failed with a typed failure_reason.
• Added classifyProRataExpenseError to distinguish deterministic failures (snapshot_currency_missing) from transient errors.
• Added ErrSnapshotCurrencyMissing and ErrMissingCapturedRateSnapshot error codes.
• Added LegacyMigration flag to CreateProRataInstallmentInput and forwarded it over gRPC.
• Updated CreatePeriodWithProRata to pass the created period object to applyPendingProRata.
• Updated all existing CreatePeriodWithProRata tests to the new CreateProRataInstallment contract.

Backend: Expense service

• Added legacy_migration field to CreateProRataInstallmentRequest proto and regenerated bindings.
• Added legacy-migration path in CreateProRataInstallment: writes a migration snapshot (exchangeRate = "1", exchangeRateSource = "migration") without calling FX.
• Forwarded LegacyMigration flag from gRPC handler to service layer.

Tests

Finance service (GOWORK=off go test ./... -count=1): all packages pass. New test coverage:

• Captured snapshot with same target currency applies with snapshot.
• Captured snapshot with different target currency applies in target currency.
• Captured snapshot missing target currency marks schedule failed.
• Transient Expense write failure leaves schedule pending.
• Legacy same-currency schedule applies with migration snapshot.
• Legacy different-currency schedule marks failed.
• No target period leaves legacy schedule pending.
• Failed schedule is not re-applied.
• Dashboard totals exclude pending and failed schedules.
• Expense snapshot_currency_missing failure classified as deterministic.
• classifyProRataExpenseError table test covering in-process and gRPC error paths.

Expense service (GOWORK=off go test ./... -count=1): all packages pass. Legacy migration and defense-in-depth validation tests added.

go vet clean for both services. Workspace build clean.

Notes

• The new MarkProRataFailed SQL query is hand-added to the sqlc-generated finance.sql.go (sqlc is not installed). It mirrors the existing MarkProRataApplied pattern. A future sqlc generate run will regenerate it identically.
• No automatic retry exists for schedules left pending by a transient failure. Application is only triggered during period creation. A manual retry endpoint is out of scope for this ticket.

References

• Ticket 14: Pro-rata future application and legacy failures
• Changeset: /Users/thompsontong/Desktop/multi-currency-support-Spec/changesets/changeset-14.md