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/constants.go

@@ -84,6 +84,13 @@ var SMTPAccount = ""
 var SMTPFrom = ""
 var SMTPToken = ""
 
+var SMSVerificationEnabled = false
+var SMSProvider = ""        // "unisms"
+var SMSAccessKeyId = ""     // UniSMS AccessKeyId
+var SMSAccessKeySecret = "" // UniSMS AccessKeySecret
+var SMSSignName = ""        // SMS signature name
+var SMSTemplateCode = ""    // SMS template code
+
 var GitHubClientId = ""
 var GitHubClientSecret = ""
 var LinuxDOClientId = ""