Switch to using tailscale oauth api

This removes the need to update the API key every 90 days.

tests/test_auth.py

@@ -5,7 +5,13 @@ client = TestClient(app)
 
 
 def get_settings_override() -> Settings:
-    return Settings(test_mode=True, tailnet="test", api_key="test", bearer_token="test")
+    return Settings(
+        test_mode=True,
+        tailnet="test",
+        client_id="test",
+        client_secret="test",
+        bearer_token="test",
+    )
 
 
 app.dependency_overrides[get_settings] = get_settings_override