refactor: review remarks

config/config.exs

@@ -104,6 +104,9 @@ config :mv, :mail_from, {"Mila", "noreply@example.com"}
 # Join form rate limiting (Hammer). scale_ms: window in ms, limit: max submits per window per IP.
 config :mv, :join_rate_limit, scale_ms: 60_000, limit: 10
 
+# Join emails: notifier implementation (domain → web abstraction). Override in test to inject a mock.
+config :mv, :join_notifier, MvWeb.JoinNotifierImpl
+
 # Configure esbuild (the version is required)
 config :esbuild,
   version: "0.17.11",