✨(backend) accept form-urlencoded on the user token endpoint

Accept `application/x-www-form-urlencoded` requests on the user
token endpoint, in addition to the existing JSON support.

This aligns the endpoint with the OAuth 2.0 specification for token
endpoint requests (RFC 6749, sections 3.2 and 4.4.2), so standard
OAuth 2.0 client libraries can call it without any customization,
while keeping backward compatibility with existing JSON clients.

CHANGELOG.md

@@ -8,6 +8,10 @@ and this project adheres to
 
 ## [Unreleased]
 
+### Changed
+
+- ✨(backend) accept form-urlencoded on the user token endpoint
+
 ### Fixed
 
 - 📝(docs) fix minor typos in comments and docstrings