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>

common/verification.go

@@ -16,6 +16,7 @@ type verificationValue struct {
 const (
 	EmailVerificationPurpose = "v"
 	PasswordResetPurpose     = "r"
+	SmsVerificationPurpose   = "s"
 )
 
 var verificationMutex sync.Mutex