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/EndpointTests.cs

@@ -101,20 +101,20 @@ public class EndpointTests(TestWebApplicationFactory factory) : IClassFixture<Te
   }
 
   [Fact]
-  public async Task Change_request_with_valid_address_succeeds()
+  public async Task Change_request_with_valid_phone_succeeds()
   {
     var res = await _client.PostAsJsonAsync("/api/v1/change-requests",
-        new { straat = "Lange Voorhout 9", postcode = "2514 EA", woonplaats = "Den Haag" });
+        new { telefoon = "0612345678" });
     res.EnsureSuccessStatusCode();
     var body = await res.Content.ReadFromJsonAsync<ReferentieResponse>();
     Assert.StartsWith("BIG-2026-", body!.Referentie);
   }
 
   [Fact]
-  public async Task Change_request_with_bad_postcode_is_rejected()
+  public async Task Change_request_with_bad_phone_is_rejected()
   {
     var res = await _client.PostAsJsonAsync("/api/v1/change-requests",
-        new { straat = "Straat 1", postcode = "nope", woonplaats = "Den Haag" });
+        new { telefoon = "nope" });
     Assert.Equal(HttpStatusCode.UnprocessableEntity, res.StatusCode);
   }