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/sms.go

@@ -0,0 +1,37 @@
+package common
+
+import (
+	"errors"
+	"regexp"
+)
+
+// SmsProvider defines the interface for SMS providers
+type SmsProvider interface {
+	SendCode(phone string, code string) error
+}
+
+var smsProviders = map[string]SmsProvider{}
+
+// RegisterSmsProvider registers an SMS provider by name
+func RegisterSmsProvider(name string, provider SmsProvider) {
+	smsProviders[name] = provider
+}
+
+// SendSMS sends a verification code via the configured SMS provider
+func SendSMS(phone string, code string) error {
+	if SMSProvider == "" {
+		return errors.New("SMS provider not configured")
+	}
+	provider, ok := smsProviders[SMSProvider]
+	if !ok {
+		return errors.New("unknown SMS provider: " + SMSProvider)
+	}
+	return provider.SendCode(phone, code)
+}
+
+var chinesePhoneRegex = regexp.MustCompile(`^1[3-9]\d{9}$`)
+
+// IsValidPhone checks if the phone number is a valid Chinese mobile number
+func IsValidPhone(phone string) bool {
+	return chinesePhoneRegex.MatchString(phone)
+}