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/Migrations/ScadaBridgeDbContextModelSnapshot.cs

@@ -824,7 +824,6 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
                     SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
 
                     b.Property<string>("EmailAddress")
-                        .IsRequired()
                         .HasMaxLength(500)
                         .HasColumnType("nvarchar(500)");
 
@@ -836,6 +835,10 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
                     b.Property<int>("NotificationListId")
                         .HasColumnType("int");
 
+                    b.Property<string>("PhoneNumber")
+                        .HasMaxLength(32)
+                        .HasColumnType("nvarchar(32)");
+
                     b.HasKey("Id");
 
                     b.HasIndex("NotificationListId");
@@ -843,6 +846,50 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
                     b.ToTable("NotificationRecipients");
                 });
 
+            modelBuilder.Entity("ZB.MOM.WW.ScadaBridge.Commons.Entities.Notifications.SmsConfiguration", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("int");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("AccountSid")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<string>("ApiBaseUrl")
+                        .HasMaxLength(500)
+                        .HasColumnType("nvarchar(500)");
+
+                    b.Property<string>("AuthToken")
+                        .HasMaxLength(8000)
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<int>("ConnectionTimeoutSeconds")
+                        .HasColumnType("int");
+
+                    b.Property<string>("FromNumber")
+                        .IsRequired()
+                        .HasMaxLength(32)
+                        .HasColumnType("nvarchar(32)");
+
+                    b.Property<int>("MaxRetries")
+                        .HasColumnType("int");
+
+                    b.Property<string>("MessagingServiceSid")
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<TimeSpan>("RetryDelay")
+                        .HasColumnType("time");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("SmsConfigurations");
+                });
+
             modelBuilder.Entity("ZB.MOM.WW.ScadaBridge.Commons.Entities.Notifications.SmtpConfiguration", b =>
                 {
                     b.Property<int>("Id")