fix: change avatar upload response to JSON object with URL;

feat: add UrlDto for standardized URL responses;
refactor: update avatar upload endpoint to return UrlDto;
docs: regenerate Swagger;
chore: add comments for untested profile controller methods

backend/docs/swagger.json

@@ -339,7 +339,7 @@
                     "multipart/form-data"
                 ],
                 "produces": [
-                    "text/plain"
+                    "application/json"
                 ],
                 "tags": [
                     "Profile"
@@ -358,7 +358,7 @@
                     "200": {
                         "description": "Uploaded image url",
                         "schema": {
-                            "type": "string"
+                            "$ref": "#/definitions/dto.UrlDto"
                         }
                     }
                 }
@@ -546,6 +546,17 @@
                 }
             }
         },
+        "dto.UrlDto": {
+            "type": "object",
+            "required": [
+                "url"
+            ],
+            "properties": {
+                "url": {
+                    "type": "string"
+                }
+            }
+        },
         "models.ChangePasswordRequest": {
             "type": "object",
             "required": [