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/Configurations/NotificationConfiguration.cs

@@ -42,9 +42,59 @@ public class NotificationRecipientConfiguration : IEntityTypeConfiguration<Notif
             .IsRequired()
             .HasMaxLength(200);
 
+        // EmailAddress is now nullable — an SMS-only recipient carries a PhoneNumber and no
+        // email. The max length is unchanged from the original email-only mapping.
         builder.Property(r => r.EmailAddress)
-            .IsRequired()
+            .IsRequired(false)
             .HasMaxLength(500);
+
+        // PhoneNumber (E.164, e.g. +14155552671) — nullable, present only for SMS recipients.
+        builder.Property(r => r.PhoneNumber)
+            .IsRequired(false)
+            .HasMaxLength(32);
+    }
+}
+
+public class SmsConfigurationConfiguration : IEntityTypeConfiguration<SmsConfiguration>
+{
+    /// <summary>Configures the EF Core mapping for <see cref="SmsConfiguration"/>.</summary>
+    /// <param name="builder">The entity type builder.</param>
+    public void Configure(EntityTypeBuilder<SmsConfiguration> builder)
+    {
+        builder.HasKey(s => s.Id);
+
+        builder.Property(s => s.AccountSid)
+            .IsRequired()
+            .HasMaxLength(100);
+
+        // Stored encrypted at rest (EncryptedStringConverter, wired in
+        // ScadaBridgeDbContext.ApplySecretColumnEncryption). Ciphertext is larger than the
+        // plaintext, so the column is sized generously to avoid truncation — mirrors
+        // SmtpConfiguration.Credentials.
+        builder.Property(s => s.AuthToken)
+            .HasMaxLength(8000);
+
+        builder.Property(s => s.FromNumber)
+            .IsRequired()
+            .HasMaxLength(32);
+
+        builder.Property(s => s.MessagingServiceSid)
+            .HasMaxLength(100);
+
+        builder.Property(s => s.ApiBaseUrl)
+            .HasMaxLength(500);
+
+        // The non-parameter constructor seeds ConnectionTimeoutSeconds/MaxRetries/RetryDelay
+        // with sensible defaults; mapping them as REQUIRED columns preserves round-trip
+        // fidelity for those values (S1 review note).
+        builder.Property(s => s.ConnectionTimeoutSeconds)
+            .IsRequired();
+
+        builder.Property(s => s.MaxRetries)
+            .IsRequired();
+
+        builder.Property(s => s.RetryDelay)
+            .IsRequired();
     }
 }