Add OpenAIKeyProvider + RealtimeSettings record

backend/Realtime/OpenAIKeyProvider.cs

@@ -0,0 +1,21 @@
+namespace backend.Realtime;
+
+public class OpenAIKeyProvider
+{
+    private readonly string? _key;
+
+    public OpenAIKeyProvider(IConfiguration cfg)
+    {
+        _key = cfg["OPENAI_API_KEY"] ?? cfg["OpenAI:ApiKey"];
+    }
+
+    public bool HasKey => !string.IsNullOrWhiteSpace(_key);
+
+    public string GetRequired()
+    {
+        if (string.IsNullOrWhiteSpace(_key))
+            throw new InvalidOperationException(
+                "OPENAI_API_KEY is not configured. Set the env var on the Coolify app.");
+        return _key!;
+    }
+}