✨(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.

docs/openapi.yaml

@@ -50,10 +50,24 @@ paths:
         
         The application must be authorized for the user's email domain.
         The returned token expires after a configured duration and must be refreshed by calling this endpoint again.
+
+        Request parameters may be sent either as "application/x-www-form-urlencoded"
+        (as specified by RFC 6749 for OAuth 2.0 token endpoints) or as "application/json".
       operationId: generateToken
       requestBody:
         required: true
         content:
+          application/x-www-form-urlencoded:
+            schema:
+              $ref: '#/components/schemas/TokenRequest'
+            examples:
+              tokenRequest:
+                summary: Request token for user delegation
+                value:
+                  client_id: "550e8400-e29b-41d4-a716-446655440000"
+                  client_secret: "1234567890abcdefghijklmnopqrstuvwxyz"
+                  grant_type: "client_credentials"
+                  scope: "user@example.com"
           application/json:
             schema:
               $ref: '#/components/schemas/TokenRequest'
@@ -117,6 +131,19 @@ paths:
                   summary: Domain not authorized
                   value:
                     error: "This application is not authorized for this email domain."
+        '415':
+          description: |
+            Unsupported media type. The request body must be sent as
+            "application/x-www-form-urlencoded" or "application/json".
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
+              examples:
+                unsupportedMediaType:
+                  summary: Unsupported request content type
+                  value:
+                    detail: 'Unsupported media type "text/plain" in request.'
 
   /rooms:
     get: