feat: initial FRED API Proxy project

- Go web server using Gin framework
- Proxy FRED economic data API
- Support series, observations, search, releases endpoints

tests/go.mod

@@ -0,0 +1,3 @@
+module fred-api-test
+
+go 1.21

tests/test_fred_api.go

@@ -0,0 +1,88 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+)
+
+const (
+	APIKey  = "987b9c3428a3d0fe90179e3e6298d05e"
+	BaseURL = "https://api.stlouisfed.org/fred"
+)
+
+func buildURL(endpoint string, params map[string]string) string {
+	u, _ := url.Parse(BaseURL + endpoint)
+	query := u.Query()
+	query.Set("api_key", APIKey)
+	query.Set("file_type", "json")
+	for k, v := range params {
+		if v != "" {
+			query.Set(k, v)
+		}
+	}
+	u.RawQuery = query.Encode()
+	return u.String()
+}
+
+func fetchAndPrint(endpoint string, params map[string]string) {
+	url := buildURL(endpoint, params)
+	fmt.Printf("\n=== Testing %s ===\n", endpoint)
+	fmt.Printf("URL: %s\n\n", url)
+
+	resp, err := http.Get(url)
+	if err != nil {
+		fmt.Printf("Error: %v\n", err)
+		return
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		fmt.Printf("Error reading body: %v\n", err)
+		return
+	}
+
+	var prettyJSON map[string]interface{}
+	if err := json.Unmarshal(body, &prettyJSON); err == nil {
+		output, _ := json.MarshalIndent(prettyJSON, "", "  ")
+		fmt.Printf("Response:\n%s\n", output)
+	} else {
+		fmt.Printf("Response:\n%s\n", string(body))
+	}
+}
+
+func main() {
+	fmt.Println("========================================")
+	fmt.Println("  FRED API 测试脚本")
+	fmt.Println("========================================")
+
+	fetchAndPrint("/series", map[string]string{"series_id": "GNPCA"})
+
+	fetchAndPrint("/series/observations", map[string]string{
+		"series_id":       "GNPCA",
+		"limit":           "5",
+		"observation_end": "2000-01-01",
+	})
+
+	fetchAndPrint("/series/search", map[string]string{
+		"search_text": "GDP",
+		"limit":       "3",
+	})
+
+	fetchAndPrint("/releases", map[string]string{
+		"limit": "3",
+	})
+
+	fetchAndPrint("/releases/dates", map[string]string{
+		"limit":      "3",
+		"start_date": "2026-03-01",
+		"end_date":   "2026-03-31",
+	})
+
+	fmt.Println("\n========================================")
+	fmt.Println("  测试完成!")
+	fmt.Println("========================================")
+}