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.yaml

@@ -471,10 +471,6 @@ paths:
           description: Presigned URL and form data
           schema:
             $ref: '#/definitions/models.PresignedUploadResponse'
-        "500":
-          description: Internal server error
-      security:
-      - JWT: []
       summary: Get presigned URL for avatar upload
       tags:
       - Upload
@@ -489,10 +485,6 @@ paths:
           description: Presigned URL and form data
           schema:
             $ref: '#/definitions/models.PresignedUploadResponse'
-        "500":
-          description: Internal server error
-      security:
-      - JWT: []
       summary: Get presigned URL for image upload
       tags:
       - Upload