feat: add SMS verification registration with UniSMS provider

- Add phone field to user model with index and helper methods
- Implement SMS provider interface with UniSMS (合一短信) implementation
- Add SMS verification code sending endpoint with rate limiting (1/60s)
- Support SMS registration in Register() (mutually exclusive with email)
- Add SMS configuration to admin settings (provider, keys, signature, template)
- Display phone number in admin user list contact column
- Add i18n translations for all SMS-related messages (zh-CN, en, zh-TW)
- Add Claude Code skills: sync-upstream, migrate-server
- Update CLAUDE.md with git conventions and deployment guide

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

i18n/keys.go

@@ -100,6 +100,12 @@ const (
 	MsgUserTelegramIdEmpty           = "user.telegram_id_empty"
 	MsgUserTelegramNotBound          = "user.telegram_not_bound"
 	MsgUserLinuxDOIdEmpty            = "user.linux_do_id_empty"
+	MsgUserSmsVerificationRequired   = "user.sms_verification_required"
+	MsgUserPhoneFormatInvalid        = "user.phone_format_invalid"
+	MsgUserPhoneAlreadyTaken         = "user.phone_already_taken"
+	MsgUserSendSmsFailed             = "user.send_sms_failed"
+	MsgUserSmsNotConfigured          = "user.sms_not_configured"
+	MsgUserPhoneEmpty                = "user.phone_empty"
 )
 
 // Quota related messages