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_settings_runtime.go

@@ -50,5 +50,5 @@ func settingsClaudeMapping(c config.Config) map[string]string {
 	if len(c.ClaudeModelMap) > 0 {
 		return c.ClaudeModelMap
 	}
-	return map[string]string{"fast": "deepseek-chat", "slow": "deepseek-reasoner"}
+	return map[string]string{"fast": "deepseek-v4-flash", "slow": "deepseek-v4-pro"}
 }