feat: remove authentication requirement for avatar and image upload endpoints;

fix: remove 500 error responses from upload endpoints;
fix: return validation error strings instead of error lists;
fix: handle invalid avatar upload IDs with 400 Bad Request response;
fix: add missing S3Controller to controller initialization;
fix: change avatar_upload_id to string type and update validation rules;
chore: add license header to smtp.go;
refactor: replace manual proxy implementation with httputil.ReverseProxy;
fix: inject S3Service dependency into ProfileService;
fix: set color and color_grad fields during profile update;
fix: correct DTO mapping for profile and settings;
fix: check object existence before copying in SaveUpload;
fix: adjust profile DTO mapping function for proper pointer handling

backend/docs/swagger.json

@@ -458,11 +458,6 @@
         },
         "/upload/avatar": {
             "get": {
-                "security": [
-                    {
-                        "JWT": []
-                    }
-                ],
                 "consumes": [
                     "application/json"
                 ],
@@ -479,20 +474,12 @@
                         "schema": {
                             "$ref": "#/definitions/models.PresignedUploadResponse"
                         }
-                    },
-                    "500": {
-                        "description": "Internal server error"
                     }
                 }
             }
         },
         "/upload/image": {
             "get": {
-                "security": [
-                    {
-                        "JWT": []
-                    }
-                ],
                 "consumes": [
                     "application/json"
                 ],
@@ -509,9 +496,6 @@
                         "schema": {
                             "$ref": "#/definitions/models.PresignedUploadResponse"
                         }
-                    },
-                    "500": {
-                        "description": "Internal server error"
                     }
                 }
             }