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/tests/BigRegister.Tests/IdempotencyTests.cs

@@ -12,7 +12,7 @@ public class IdempotencyTests(TestWebApplicationFactory factory) : IClassFixture
   {
     var req = new HttpRequestMessage(HttpMethod.Post, "/api/v1/change-requests")
     {
-      Content = JsonContent.Create(new { straat = "Lange Voorhout 9", postcode = "2514 EA", woonplaats = "Den Haag" }),
+      Content = JsonContent.Create(new { telefoon = "0612345678" }),
     };
     req.Headers.Add("Idempotency-Key", key);
     return req;
@@ -51,7 +51,7 @@ public class IdempotencyTests(TestWebApplicationFactory factory) : IClassFixture
     var key = Guid.NewGuid().ToString();
     var badRequest = new HttpRequestMessage(HttpMethod.Post, "/api/v1/change-requests")
     {
-      Content = JsonContent.Create(new { straat = "Straat 1", postcode = "nope", woonplaats = "Den Haag" }),
+      Content = JsonContent.Create(new { telefoon = "nope" }),
     };
     badRequest.Headers.Add("Idempotency-Key", key);
 
@@ -60,7 +60,7 @@ public class IdempotencyTests(TestWebApplicationFactory factory) : IClassFixture
 
     var replayRequest = new HttpRequestMessage(HttpMethod.Post, "/api/v1/change-requests")
     {
-      Content = JsonContent.Create(new { straat = "Straat 1", postcode = "nope", woonplaats = "Den Haag" }),
+      Content = JsonContent.Create(new { telefoon = "nope" }),
     };
     replayRequest.Headers.Add("Idempotency-Key", key);
     var replay = await _client.SendAsync(replayRequest);