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/rawsample/rawsample_test.go

@@ -22,7 +22,7 @@ func TestPersistWritesSampleFilesAndMeta(t *testing.T) {
 		SampleID: "My Sample! 01",
 		Source:   "unit-test",
 		Request: map[string]any{
-			"model":  "deepseek-chat",
+			"model":  "deepseek-v4-flash",
 			"stream": true,
 			"messages": []any{
 				map[string]any{"role": "user", "content": "广州天气"},