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/20260619135543_AddSmsNotifications.cs

@@ -0,0 +1,68 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
+{
+    /// <summary>
+    /// SMS Notifications (S2): makes a notification recipient's contact path optional and adds
+    /// the central SMS-provider (Twilio) configuration table.
+    ///
+    /// 1. <c>NotificationRecipients.EmailAddress</c> becomes nullable — an SMS-only recipient
+    ///    carries a <c>PhoneNumber</c> and no email. (ALTER COLUMN, naturally idempotent.)
+    /// 2. <c>NotificationRecipients.PhoneNumber nvarchar(32) NULL</c> — E.164 phone for SMS
+    ///    recipients (guarded ADD).
+    /// 3. <c>SmsConfigurations</c> table — central-only provider config; <c>AuthToken</c> is
+    ///    stored encrypted at rest (EncryptedStringConverter → <c>nvarchar(max)</c>, mirroring
+    ///    <c>SmtpConfigurations.Credentials</c>) (guarded CREATE TABLE).
+    ///
+    /// Every DDL statement is guarded so the migration is safe to re-run against a
+    /// partially-migrated database (the repo's idempotent-migration convention; see
+    /// AddListAttributeElementType).
+    /// </summary>
+    public partial class AddSmsNotifications : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            // ALTER COLUMN is naturally idempotent: re-running re-applies the same nullable shape.
+            migrationBuilder.Sql("ALTER TABLE [NotificationRecipients] ALTER COLUMN [EmailAddress] nvarchar(500) NULL;");
+
+            migrationBuilder.Sql(@"
+IF NOT EXISTS (SELECT 1 FROM sys.columns WHERE Name='PhoneNumber' AND Object_ID=Object_ID('NotificationRecipients'))
+    ALTER TABLE [NotificationRecipients] ADD [PhoneNumber] nvarchar(32) NULL;");
+
+            migrationBuilder.Sql(@"
+IF OBJECT_ID(N'dbo.SmsConfigurations', N'U') IS NULL
+    CREATE TABLE [SmsConfigurations] (
+        [Id] int NOT NULL IDENTITY(1, 1),
+        [AccountSid] nvarchar(100) NOT NULL,
+        [AuthToken] nvarchar(max) NULL,
+        [FromNumber] nvarchar(32) NOT NULL,
+        [MessagingServiceSid] nvarchar(100) NULL,
+        [ApiBaseUrl] nvarchar(500) NULL,
+        [ConnectionTimeoutSeconds] int NOT NULL,
+        [MaxRetries] int NOT NULL,
+        [RetryDelay] time NOT NULL,
+        CONSTRAINT [PK_SmsConfigurations] PRIMARY KEY ([Id])
+    );");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.Sql(@"
+IF OBJECT_ID(N'dbo.SmsConfigurations', N'U') IS NOT NULL
+    DROP TABLE [SmsConfigurations];");
+
+            migrationBuilder.Sql(@"
+IF EXISTS (SELECT 1 FROM sys.columns WHERE Name='PhoneNumber' AND Object_ID=Object_ID('NotificationRecipients'))
+    ALTER TABLE [NotificationRecipients] DROP COLUMN [PhoneNumber];");
+
+            // Reversing to NOT NULL requires backfilling existing NULLs first, otherwise the
+            // ALTER fails. Mirrors EF's generated default ('') for the previously-required column.
+            migrationBuilder.Sql("UPDATE [NotificationRecipients] SET [EmailAddress] = '' WHERE [EmailAddress] IS NULL;");
+            migrationBuilder.Sql("ALTER TABLE [NotificationRecipients] ALTER COLUMN [EmailAddress] nvarchar(500) 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")