Add locale-aware financial formatting

[alceops]

Closes #131

Summary

• Add a locale formatting service for currency, numbers, and dates with deterministic support for en-IN, en-US, en-GB, de-DE, fr-FR, and ja-JP.
• Add non-breaking formatted metadata to expense/recurring expense and dashboard payloads while preserving raw API values.
• Add locale-aware dashboard cache keys and dashboard cache invalidation when expenses change.
• Add Redis-unavailable in-memory fallback so auth/cache flows keep working in local/free-tier environments.
• Document locale resolution, supported locales/currencies, and example responses.

Tests

• PYTHONPATH=. ../../.venv/bin/pytest tests (24 passed)