♻️(backend) refactor uploaded file's key format

Remove the user-provided filename while preserving the extension,
and switch to the format `uuid.extension`.

This is more natural than the previous `uuid/.extension` format
and avoids confusion.

Update related tests accordingly.

src/backend/core/api/viewsets.py

@@ -85,7 +85,7 @@ UUID_REGEX = (
 FILE_EXT_REGEX = r"[\d\w]+"
 MEDIA_STORAGE_URL_PATTERN = re.compile(
     f"{settings.MEDIA_URL:s}"
-    rf"(?P<key>{FILE_FOLDER:s}/(?P<pk>{UUID_REGEX:s})/\.{FILE_EXT_REGEX:s})$"
+    rf"(?P<key>{FILE_FOLDER:s}/(?P<pk>{UUID_REGEX:s})\.{FILE_EXT_REGEX:s})$"
 )