feat(notifications): central SMS config + nullable recipient contact (S2)

Implement the central ConfigurationDatabase side of SMS notifications:

- NotificationConfiguration: EmailAddress now nullable (SMS-only recipients
  carry a PhoneNumber, no email); add PhoneNumber nvarchar(32); add
  SmsConfigurationConfiguration (AuthToken sized as the encrypted column,
  mirroring SmtpConfiguration.Credentials; timeout/retry mapped REQUIRED for
  ctor-default round-trip fidelity).
- ScadaBridgeDbContext: add SmsConfigurations DbSet, encrypt AuthToken at rest
  via EncryptedStringConverter, and cover SmsConfiguration in the schema-only
  secret-write guard.
- NotificationRepository: implement the four INotificationRepository SMS-config
  methods (resolves the 4x CS0535), mirroring the SMTP methods' stage-only /
  separate-SaveChangesAsync discipline.
- Migration AddSmsNotifications: idempotent (guarded) ALTER EmailAddress nullable,
  ADD PhoneNumber, CREATE SmsConfigurations; Down reverses cleanly (backfills
  NULL emails before restoring NOT NULL).

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Repositories/NotificationRepository.cs

@@ -88,6 +88,22 @@ public class NotificationRepository : INotificationRepository
         if (entity != null) _context.Set<SmtpConfiguration>().Remove(entity);
     }
 
+    /// <inheritdoc />
+    public async Task<SmsConfiguration?> GetSmsConfigurationAsync(CancellationToken cancellationToken = default)
+        => await _context.Set<SmsConfiguration>().FirstOrDefaultAsync(cancellationToken);
+
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<SmsConfiguration>> GetAllSmsConfigurationsAsync(CancellationToken cancellationToken = default)
+        => await _context.Set<SmsConfiguration>().ToListAsync(cancellationToken);
+
+    /// <inheritdoc />
+    public async Task AddSmsConfigurationAsync(SmsConfiguration smsConfiguration, CancellationToken cancellationToken = default)
+        => await _context.Set<SmsConfiguration>().AddAsync(smsConfiguration, cancellationToken);
+
+    /// <inheritdoc />
+    public Task UpdateSmsConfigurationAsync(SmsConfiguration smsConfiguration, CancellationToken cancellationToken = default)
+    { _context.Set<SmsConfiguration>().Update(smsConfiguration); return Task.CompletedTask; }
+
     /// <inheritdoc />
     public async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
         => await _context.SaveChangesAsync(cancellationToken);