[BE-93] Wire up nestjs-i18n or remove it (currently an unused dependency)

[yusuftomilola]

Overview

nestjs-i18n and i18n are declared dependencies but imported in zero files (grep -rl nestjs-i18n backend/src → 0). All API messages are hardcoded English strings.

Tasks

• Decide with maintainers whether multi-language support is in scope.
• If yes: register I18nModule with a src/i18n/ translation directory, resolve locale from Accept-Language and an optional user preference, and migrate validation and error messages to translation keys.
• Translate at minimum the transactional email templates and auth/validation errors.
• If no: remove nestjs-i18n and i18n from package.json.

Acceptance Criteria

• Either Accept-Language: fr measurably changes API messages, or the dependencies are gone.
• No unused i18n packages remain in package.json.

Notes for Contributors

Comment below to be assigned.