feat: i18n support via Accept-Language header (en, id)

[zulfikar-ditya]

Summary

• Implements an AsyncLocalStorage-backed translation layer (t() + translateValidationIssue) sourced from per-locale JSON dictionaries and an auto-generated TranslationKey union (bun run i18n:keys).
• Locale is selected per request from the Accept-Language header (default en), echoed back via Content-Language, propagated through every route handler, service, error, validation message, and the BullMQ email pipeline (lang is stamped on the job payload and the worker re-enters the locale before rendering).
• Bilingual email templates (<template>.<locale>.html with English fallback), Accept-Language documented globally in the OpenAPI/Scalar spec, and bun run i18n:keys is now in the pre-commit hook so missing/extra id.json keys fail commits.

Test plan

• bun run typecheck and bun run lint are clean.
• Hit any endpoint with Accept-Language: id and confirm the success/error message and Content-Language: id header.
• Trigger a 422 validation error (e.g. invalid email on /auth/register) with Accept-Language: id and confirm field-level messages render in Indonesian.
• Register a user with Accept-Language: id and confirm the verification email uses email-verification.id.html and the Indonesian subject.
• Open /reference (Scalar) and confirm every route lists the Accept-Language header parameter.
• Add a key to en.json without mirroring it in id.json and confirm bun run i18n:keys exits non-zero (pre-commit will catch it).

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