refactor: address review comments for join view

lib/mv_web/join_rate_limit.ex

@@ -15,6 +15,7 @@ defmodule MvWeb.JoinRateLimit do
   - `{:deny, _retry_after_ms}` - rate limit exceeded
   """
   def check(key) when is_binary(key) do
+    # Read at runtime so config can be changed without restart (e.g. in tests).
     config = Application.get_env(:mv, :join_rate_limit, [])
     scale_ms = Keyword.get(config, :scale_ms, 60_000)
     limit = Keyword.get(config, :limit, 10)