feat(api): 接入微信登录并原生实现咨询域（ECR-049/050）

小程序可在 Go 上完成微信手机号登录、测评、预约和下单，不再反代 Java。

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/internal/wechatpay/sign_test.go

@@ -0,0 +1,17 @@
+package wechatpay
+
+import (
+	"crypto/md5"
+	"encoding/hex"
+	"strings"
+	"testing"
+)
+
+func TestSignMD5(t *testing.T) {
+	got := SignMD5(map[string]string{"appid": "wx123", "mch_id": "foo", "sign": "x"}, "k")
+	sum := md5.Sum([]byte("appid=wx123&mch_id=foo&key=k"))
+	want := strings.ToUpper(hex.EncodeToString(sum[:]))
+	if got != want {
+		t.Fatalf("got %s want %s", got, want)
+	}
+}