feat: implement SMS OTP auth via backend API

- signInWithPhoneNumber: calls POST /auth/send-otp, stores pending phone
- verifyOTP: calls POST /auth/phone with phone+code, saves token
- linkWithOTP: calls POST /auth/verify-phone (authenticated) with phone+code
- AuthBloc._linkWithPhoneNumber: replaced Firebase PhoneVerificationService with repository call

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

lib/blocs/auth_bloc/auth_bloc.dart

@@ -9,9 +9,6 @@ part 'auth_state.dart';
 
 class AuthBloc extends Bloc<AuthEvent, AuthState> {
   final UserRepository _userRepository;
-  final PhoneVerificationService phoneVerificationService =
-      PhoneVerificationService();
-  String? _verificationId;
 
   AuthBloc({required UserRepository userRepository})
       : _userRepository = userRepository,
@@ -103,39 +100,11 @@ class AuthBloc extends Bloc<AuthEvent, AuthState> {
   void _linkWithPhoneNumber(
       LinkWithPhoneNumber event, Emitter<AuthState> emit) async {
     emit(AuthStateProcess());
-
     try {
-      await for (PhoneAuthEvent event
-          in phoneVerificationService.verifyPhoneNumber(event.phoneNumber)) {
-        switch (event.type) {
-          case PhoneAuthEventType.verificationCompleted:
-            final credential = event.data;
-            print('Verificacion completada. Credencial: $credential');
-            break;
-          case PhoneAuthEventType.verificationFailed:
-            final exception = event.data;
-            print('Verificacion fallida. Excepcion: $exception');
-            emit(AuthStateFailure(message: "Error inesperado. $exception"));
-            return;
-          case PhoneAuthEventType.codeAutoRetrievalTimeout:
-            final verificationId = event.data as String;
-            print('Tiempo de espera agotado. ID: $verificationId');
-            emit(const AuthStateFailure(
-              message: "Tiempo de espera agotado.",
-            ));
-            break;
-          case PhoneAuthEventType.codeSent:
-            final eventData = event.data as Map<String, dynamic>;
-            final verificationId = eventData['verificationId'] as String;
-            final resendToken = eventData['resendToken'] as int?;
-            print('Codigo enviado. ID: $verificationId, resendToken: $resendToken');
-            _verificationId = verificationId;
-            emit(const AuthStateVerifyOAuth(false));
-            break;
-        }
-      }
+      await _userRepository.signInWithPhoneNumber(event.phoneNumber);
+      emit(const AuthStateVerifyOAuth(false));
     } catch (e) {
-      emit(const AuthStateFailure(message: "Error inesperado."));
+      emit(AuthStateFailure(message: "Error al enviar código. $e"));
     }
   }
 
@@ -144,8 +113,7 @@ class AuthBloc extends Bloc<AuthEvent, AuthState> {
     emit(AuthStateProcess());
     try {
       final bool isVerified = await _userRepository.linkWithOTP(
-          event.phoneNumber, _verificationId ?? '', event.code);
-
+          event.phoneNumber, '', event.code);
       if (isVerified) {
         emit(AuthStateSuccess());
       } else {