♻️(backend) improve scope prefix removal logic

The previous replace usage was too broad and could remove multiple
occurrences, which was not the original intention.

Replace the replace call with removeprefix, which more accurately
matches the expected behavior by only removing the prefix when present
at the start of the string.

src/backend/core/external_api/permissions.py

@@ -59,7 +59,7 @@ class BaseScopePermission(permissions.BasePermission):
 
         if settings.OIDC_RS_SCOPES_PREFIX:
             token_scopes = [
-                scope.replace(f"{settings.OIDC_RS_SCOPES_PREFIX}:", "")
+                scope.removeprefix(f"{settings.OIDC_RS_SCOPES_PREFIX}:")
                 for scope in token_scopes
             ]