Use User.ID instead of User.Name in ActivityPub API for Person IRI (#23823)

Thanks to @trwnh

Close #23802

The ActivityPub id is an HTTPS URI that should remain constant, even if
the user changes their name.

templates/swagger/v1_json.tmpl

@@ -23,7 +23,7 @@
   },
   "basePath": "{{AppSubUrl | JSEscape | Safe}}/api/v1",
   "paths": {
-    "/activitypub/user/{username}": {
+    "/activitypub/user-id/{user-id}": {
       "get": {
         "produces": [
           "application/json"
@@ -35,9 +35,9 @@
         "operationId": "activitypubPerson",
         "parameters": [
           {
-            "type": "string",
-            "description": "username of the user",
-            "name": "username",
+            "type": "integer",
+            "description": "user ID of the user",
+            "name": "user-id",
             "in": "path",
             "required": true
           }
@@ -49,7 +49,7 @@
         }
       }
     },
-    "/activitypub/user/{username}/inbox": {
+    "/activitypub/user-id/{user-id}/inbox": {
       "post": {
         "produces": [
           "application/json"
@@ -61,9 +61,9 @@
         "operationId": "activitypubPersonInbox",
         "parameters": [
           {
-            "type": "string",
-            "description": "username of the user",
-            "name": "username",
+            "type": "integer",
+            "description": "user ID of the user",
+            "name": "user-id",
             "in": "path",
             "required": true
           }