feat: add Solana payment scanning with USDT/USDC support and wallet management API

- Rewrite Solana scanner: stateless scan with processed-signature cache, query wallet + USDT ATA + USDC ATA addresses to cover all SPL transfers
  - Add network field to transaction_lock (4-tuple unique index) and order flow
  - Add wallet management endpoints (add/list/get/status/delete) with token auth
  - Add network info to Telegram payment notification
  - Add USDC hint to test script GMPay prompt
  - Add API docs and Solana scanning docs

src/middleware/check_token.go

@@ -0,0 +1,24 @@
+package middleware
+
+import (
+	"github.com/assimon/luuu/config"
+	"github.com/assimon/luuu/util/constant"
+	"github.com/labstack/echo/v4"
+)
+
+// CheckApiToken validates the Authorization header against the configured api_auth_token.
+// Use this for management APIs (GET/POST) where body-based signature is not practical.
+func CheckApiToken() echo.MiddlewareFunc {
+	return func(next echo.HandlerFunc) echo.HandlerFunc {
+		return func(ctx echo.Context) error {
+			token := ctx.Request().Header.Get("Authorization")
+			if token == "" {
+				token = ctx.QueryParam("api_token")
+			}
+			if token == "" || token != config.GetApiAuthToken() {
+				return constant.SignatureErr
+			}
+			return next(ctx)
+		}
+	}
+}