feat(registratie): WP-34 — phone field + BRP address read-only

Reshape the adreswijziging form into a contact-change form: the BRP address is
authoritative and shown read-only (you change it at the gemeente), and the phone
number becomes the editable/submittable field. New Telefoonnummer value object
(parse-don't-validate); backend RejectPhoneChange re-validates as authority.
POST /change-requests now carries { telefoon } (typed client regenerated).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/BigRegister.Api/Program.cs

@@ -146,7 +146,7 @@ api.MapPost("/intakes", (IntakeRequest req, HttpContext ctx) =>
 .ProducesProblem(StatusCodes.Status422UnprocessableEntity);
 
 api.MapPost("/change-requests", (ChangeRequestRequest req, HttpContext ctx) =>
-    Submit(ctx, "adreswijziging", SubmissionRules.RejectChangeRequest(req.Straat, req.Postcode)))
+    Submit(ctx, "telefoonwijziging", SubmissionRules.RejectPhoneChange(req.Telefoon)))
 .Produces<ReferentieResponse>()
 .ProducesProblem(StatusCodes.Status422UnprocessableEntity);