feat: get llm model from env

Author: pashadia

src/server.rs

@@ -225,8 +225,10 @@ impl RustDocsServer {
                         doc.content, question
                     );
 
+                    let llm_model: String = env::var("LLM_MODEL")
+                        .unwrap_or_else(|_| "gpt-4o-mini-2024-07-18".to_string());
                     let chat_request = CreateChatCompletionRequestArgs::default()
-                        .model("gpt-4o-mini-2024-07-18")
+                        .model(llm_model)
                         .messages(vec![
                             ChatCompletionRequestSystemMessageArgs::default()
                                 .content(system_prompt)