feat: accept DISCOURSE_KEY as env alias for the API key

NewFromEnv now falls back to DISCOURSE_KEY when DISCOURSE_API_KEY
is unset (the explicit name still wins), matching the shorter
DISCOURSE_URL/DISCOURSE_KEY convention used across the MOPAC fleet
scripts. Behavioral test proves both the alias and the precedence
against the fake server.

Part of Redmine 507 (Discourse Go client).

discourse.go

@@ -117,11 +117,16 @@ func New(baseURL, apiKey, apiUsername string) (*Client, error) {
 	}, nil
 }
 
-// NewFromEnv builds a client from DISCOURSE_URL, DISCOURSE_API_KEY and
-// DISCOURSE_API_USERNAME (default "system"). The intended source is a
-// 0600 env file (see env.example), sourced before the process starts.
+// NewFromEnv builds a client from DISCOURSE_URL, the API key
+// (DISCOURSE_API_KEY, falling back to the shorter DISCOURSE_KEY alias)
+// and DISCOURSE_API_USERNAME (default "system"). The intended source is
+// a 0600 env file (see env.example), sourced before the process starts.
 func NewFromEnv() (*Client, error) {
-	return New(os.Getenv("DISCOURSE_URL"), os.Getenv("DISCOURSE_API_KEY"), os.Getenv("DISCOURSE_API_USERNAME"))
+	key := os.Getenv("DISCOURSE_API_KEY")
+	if key == "" {
+		key = os.Getenv("DISCOURSE_KEY")
+	}
+	return New(os.Getenv("DISCOURSE_URL"), key, os.Getenv("DISCOURSE_API_USERNAME"))
 }
 
 // String renders the client for logs: base url + acting username only.