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/swagger.json

@@ -1528,15 +1528,7 @@
       "ChangeRequestRequest": {
         "type": "object",
         "properties": {
-          "straat": {
-            "type": "string",
-            "nullable": true
-          },
-          "postcode": {
-            "type": "string",
-            "nullable": true
-          },
-          "woonplaats": {
+          "telefoon": {
             "type": "string",
             "nullable": true
           }