Make AWS S3 bucket name fully configurable

Describe the issue
Currently, the AWS S3 bucket name is constructed dynamically in code like this:

return f"{self.AWS_S3_BUCKET_PREFIX}-{self.ENVIRONMENT}"

• This assumes that all bucket names will follow the - pattern.

• However, in future scenarios, users or organizations might bring their own bucket names, which may not include the environment in their naming convention or may follow completely different patterns.

• Hardcoding the bucket name composition like this makes it inflexible and prone to failure if the naming convention does not match.

Expected behavior

• There should be a single, fully-configurable AWS S3 bucket name setting(e.g., AWS_S3_BUCKET_NAME)which is used directly without concatenating prefix and environment in code.

• This allows using existing customer buckets regardless of naming convention.

在 backend 中搜索 issue 所示的基于 AWS_S3_BUCKET_PREFIX 和 ENVIRONMENT 的 bucket 构造方式，然后追踪 bucket 设置的读取位置。将这种组合替换为单一的可配置 bucket 名称设置，并验证使用的是未作任何修改的已配置名称，包括不带环境后缀的名称。