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/config/config.go

@@ -328,3 +328,11 @@ func GetCallbackRetryBaseDuration() time.Duration {
 	}
 	return time.Duration(seconds) * time.Second
 }
+
+func GetSolanaRpcUrl() string {
+	rpcUrl := viper.GetString("solana_rpc_url")
+	if rpcUrl == "" {
+		return "https://api.mainnet-beta.solana.com"
+	}
+	return rpcUrl
+}