🐛(backend) add AWS_S3_SIGNATURE_VERSION environment variable support

Add support for the `AWS_S3_SIGNATURE_VERSION` environment variable to
allow configuring S3 signature version for compatibility with
S3-compatible storage services like Linode Object Storage.

Fixes #1788

Signed-off-by: dtinth on MBP M1 <dtinth@spacet.me>

src/backend/impress/settings.py

@@ -169,6 +169,11 @@ class Base(Configuration):
         environ_name="AWS_STORAGE_BUCKET_NAME",
         environ_prefix=None,
     )
+    AWS_S3_SIGNATURE_VERSION = values.Value(
+        "s3v4",
+        environ_name="AWS_S3_SIGNATURE_VERSION",
+        environ_prefix=None,
+    )
 
     # Document images
     DOCUMENT_IMAGE_MAX_SIZE = values.IntegerValue(