collab: Restrict usage of the LLM service to accounts older than 30 days (#16133)

This PR restricts usage of the LLM service to accounts older than 30
days.

We now store the GitHub user's `created_at` timestamp to check the
GitHub account age. If this is not set—which it won't be for existing
users—then we use the `created_at` timestamp in the Zed database.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

crates/collab/src/rpc.rs

@@ -4907,6 +4907,9 @@ async fn accept_terms_of_service(
     Ok(())
 }
 
+/// The minimum account age an account must have in order to use the LLM service.
+const MIN_ACCOUNT_AGE_FOR_LLM_USE: chrono::Duration = chrono::Duration::days(30);
+
 async fn get_llm_api_token(
     _request: proto::GetLlmToken,
     response: Response<proto::GetLlmToken>,
@@ -4928,6 +4931,14 @@ async fn get_llm_api_token(
         Err(anyhow!("terms of service not accepted"))?
     }
 
+    let mut account_created_at = user.created_at;
+    if let Some(github_created_at) = user.github_user_created_at {
+        account_created_at = account_created_at.min(github_created_at);
+    }
+    if Utc::now().naive_utc() - account_created_at < MIN_ACCOUNT_AGE_FOR_LLM_USE {
+        Err(anyhow!("account too young"))?
+    }
+
     let token = LlmTokenClaims::create(
         user.id,
         session.is_staff(),