feat(sms): make FromNumber optional — support Twilio Messaging-Service-only configs (UI-Med-2)

Code-review finding UI-Med-2: the design doc + delivery adapter treat FromNumber and
MessagingServiceSid as either-or, but the entity ctor, EF schema, UI and CLI all hard-
required FromNumber — so a Messaging-Service-only Twilio config (a normal production
setup) could not be created. Bring the implementation into line with the spec:

- Commons: SmsConfiguration.FromNumber -> string? (ctor fromNumber optional);
  UpdateSmsConfigCommand.FromNumber -> string?.
- ConfigurationDatabase: FromNumber.IsRequired(false) + migration SmsFromNumberOptional
  (ALTER COLUMN nullable, idempotent; Down backfills '' — harmless, MsgSid keeps it
  deliverable) + regenerated model snapshot.
- Transport: SmsConfigDto.FromNumber -> string? (round-trips a Messaging-Service-only config).
- CentralUI: form validation requires AccountSid + at-least-one-of(FromNumber, MsgSid);
  nullable create/edit paths; From-number help text.
- CLI: --from-number no longer Required; BuildUpdateSmsConfigCommand validates the either-or.
- Adapter: From branch null-forgiving (guarded by the existing incomplete-config check).

Tests: ManagementActor MsgSid-only persists null FromNumber; CLI MsgSid-only builds +
neither-throws + contract (--from-number not Required); CentralUI MsgSid-only save.

src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/Delivery/SmsNotificationDeliveryAdapter.cs

@@ -211,7 +211,9 @@ public sealed class SmsNotificationDeliveryAdapter : INotificationDeliveryAdapte
             }
             else
             {
-                form.Add(new KeyValuePair<string, string>("From", smsConfig.FromNumber));
+                // The line 113 guard guarantees a non-null FromNumber whenever there is no
+                // MessagingServiceSid, so this branch never sends a null From.
+                form.Add(new KeyValuePair<string, string>("From", smsConfig.FromNumber!));
             }
 
             using var request = new HttpRequestMessage(HttpMethod.Post, requestUri)