feat(auth): add MizbanSMS OTP provider with per-flow messages and signup recovery

- Add MizbanSMS as selectable SMS provider (SMS_PROVIDER), keep Kavenegar
- Distinct OTP wording per flow via OtpMessageKind (register/login/change-phone)
- register() resumes an unverified account instead of blocking re-registration
- Hourly cleanup of abandoned unverified accounts (>24h) + expired OTP codes

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/common/enums.ts

@@ -15,6 +15,20 @@ export enum VerificationPurpose {
   CHANGE_PHONE = 'change_phone',
 }
 
+/**
+ * Which OTP wording to send. Registration and passwordless login share the
+ * LOGIN verification purpose but want different message text, so the message
+ * kind is tracked separately from the purpose.
+ */
+export enum OtpMessageKind {
+  /** First-time signup / completing verification of an unverified account. */
+  REGISTER = 'register',
+  /** Passwordless one-time-password login for a verified account. */
+  LOGIN = 'login',
+  /** Confirming a new phone number from the account page. */
+  CHANGE_PHONE = 'change_phone',
+}
+
 export enum TicketDepartment {
   TECHNICAL = 'technical',
   SALES = 'sales',