feat: revamp DeepSeek v4 model handling

- replace legacy DeepSeek ids with the new deepseek-v4 model family\n- move thinking control to request parameters and preserve assistant reasoning content\n- switch history split to IGNORE transcript injection and map upload auth failures to 401\n- update admin defaults, API docs, samples, and tests for the new model scheme

internal/admin/handler_raw_samples.go

@@ -114,7 +114,7 @@ func prepareRawSampleCaptureRequest(store ConfigStore, req map[string]any) (map[
 	}
 
 	if model := strings.TrimSpace(fieldString(payload, "model")); model == "" {
-		payload["model"] = "deepseek-chat"
+		payload["model"] = "deepseek-v4-flash"
 	}
 	if _, ok := payload["stream"]; !ok {
 		payload["stream"] = true