feat: add regex fallback for verification code extraction without Workers AI
When AI email extraction is enabled but no Workers AI binding is available,
fall back to a built-in, zero-dependency regex extractor so self-hosted
deployments without Workers AI still surface verification codes in Telegram
notifications and webhooks.
- Add worker/src/email/extract_code.ts: rule-based multilingual
(English / Chinese / Japanese / Korean) verification-code extractor with
year and YYYYMMDD date rejection to avoid false positives.
- ai_extract.ts: share the allowlist check and content parsing across both
paths, extract a saveExtractMetadata helper, and use the regex fallback
when env.AI is absent.
- Reuse the existing aiExtractResult pipeline (auth_code type), so Telegram
and webhook output need no changes.
- Update bilingual CHANGELOG and AI-extract feature docs.
