Multi-currency: documentation and rollout readiness

[ItsThompson]

Summary

Updated project documentation so it describes the shipped multi-currency model instead of the previous single-currency design. All docs now name the FX Service and its internal gRPC boundaries, period reporting currency, expense money snapshots, the startup backfill, pro-rata captured snapshots, the enriched export schema, FX configuration, FX metrics and alert thresholds, and the legacy currency alias removal.

What changed

Documentation

• docs/architecture.md: added FX Service to the node topology, dependency edges, service responsibilities, network-isolation table, and design decisions. Updated expense, finance, and datarights responsibilities and the expense/pro-rata data-flow sequence diagrams. Stated that the gateway has no /api/fx prefix.
• docs/data-model.md: added the multi-currency model overview, period reporting_currency with future-scoped default settings, expense money snapshots and the startup backfill, pro-rata captured snapshots with failure reasons, the shared currency catalog, and historical migration semantics. Added FX Service to the database ownership table.
• docs/api.md: added the Multi-Currency Fields and Compatibility section: reporting and transaction currency, the legacy currency alias removal, period context behavior, correction currency behavior, pro-rata period fields, and FX error responses. Updated the expenses/finance endpoint sections and the error-category table.
• docs/data-export.md: replaced legacy expense and period columns with the shipped transaction/reporting snapshot columns. Added reporting_currency to budget_periods.csv and currency precision formatting rules. Updated formatting helpers to formatMinorUnits.
• docs/monitoring.md: added the FX Service metrics table, export_currency_formatting_fallback_total, and an FX/multi-currency alert-context section with wired-vs-defined status.
• docs/development.md: added FX Service to key variable groups.

Configuration

• .env.example: added OPEN_EXCHANGE_RATES_APP_ID, FX_PROVIDER_TIMEOUT, FX_PROVIDER_RETRY_COUNT, FX_CACHE_MAX_AGE, and FX_SERVICE_ADDR without real secrets.
• docker-compose.yml: added a comment above fx-service identifying it as an internal-only compute service with no browser route and no gateway prefix.
• docker-compose.dev.yml: added an internal-only pointer comment referencing the main compose file.

Tests

• docker compose config --quiet: compose configuration validated.
• just test-monitoring: all three prometheus rule files valid, both promtool rule tests passed.
• git diff --check: clean, no whitespace errors.
• No Go or frontend code changed; backend and frontend test suites are unaffected.

Notes

• FX alert thresholds are documented as "defined, not wired" rather than implemented as rules. Only ServiceDown is active today for fx-service (inherited via its scrape target).
• FX_SERVICE_ADDR is documented in .env.example but remains hardcoded in docker-compose.yml, matching the existing convention for all service addresses.

References

• Ticket: "Multi-currency: documentation and rollout readiness" (feat(datarights): add GDPR data export service with email delivery  #15)