Fix VPN verify: via-env→via-file, add pass_len debug logging

OpenVPN 2.7 Windows DCO passes username but NOT password as
environment variable with via-env mode. Switched to via-file:
  server.conf: auth-user-pass-verify script via-file
  verify.sh reads credentials from temp file ():
    line 1 = username, line 2 = password

Also added pass_len to debug log so we can immediately see
if the password was actually received by the script.

backend/pkg/openvpn/manager.go

@@ -171,13 +171,13 @@ func (m *Manager) WriteServerConf(in *model.Instance, extraDir string) error {
 	}
 	if in.AuthMode == model.AuthCertPassword {
 		// 双因素认证:证书 + 用户名密码
-		// OpenVPN 以 via-env 方式调用 verify 脚本,环境变量:
+		// OpenVPN 以 via-file 方式调用 verify 脚本,$1 为临时文件路径:
-		//   username=<VPN用户名(=证书CN)>
+		//   第 1 行 = VPN用户名(=证书CN)
-		//   password=<用户输入的明文密码>
+		//   第 2 行 = 用户输入的明文密码
 		// 脚本退出码 0 = 允许,非 0 = 拒绝
 		verifyScript := filepath.Join(m.InstanceDir(in.Name), "verify.sh")
 		conf.WriteString("script-security 2\n")
-		conf.WriteString("auth-user-pass-verify \"" + verifyScript + "\" via-env\n")
+		conf.WriteString("auth-user-pass-verify \"" + verifyScript + "\" via-file\n")
 		conf.WriteString("verify-client-cert require\n")
 		_ = m.WriteVerifyScriptWithPort(in.Name, verifyScript, m.port)
 	}
@@ -507,16 +507,25 @@ func (m *Manager) WriteVerifyScript(instanceName, scriptPath string) error {
 // WriteVerifyScriptWithPort 生成带指定端口的认证脚本。
 func (m *Manager) WriteVerifyScriptWithPort(instanceName, scriptPath string, port int) error {
 	script := fmt.Sprintf(`#!/bin/bash
-# OpenVPN auth-user-pass-verify script (via-env).
+# OpenVPN auth-user-pass-verify script (via-file).
 # Generated by openvpn-manager — do NOT edit manually.
-# Calls manager API to verify bcrypt password. Exit 0=allow, 1=deny.
+# OpenVPN writes credentials to a temp file passed as $1:
+#   line 1 = username, line 2 = password
 LOG="/tmp/openvpn-verify.log"
 PORT=%d
 INSTANCE="%s"
+CREDS_FILE="$1"
 
-user="${username:-}"
+echo "$(date '+%%Y-%%m-%%d %%H:%%M:%%S') VERIFY instance=$INSTANCE creds_file=$CREDS_FILE" >> "$LOG"
-pass="${password:-}"
+
-echo "$(date '+%%Y-%%m-%%d %%H:%%M:%%S') VERIFY user=$user instance=$INSTANCE" >> "$LOG"
+if [ -z "$CREDS_FILE" ] || [ ! -f "$CREDS_FILE" ]; then
+  echo "$(date '+%%Y-%%m-%%d %%H:%%M:%%S') DENY no credentials file" >> "$LOG"
+  exit 1
+fi
+
+user=$(sed -n '1p' "$CREDS_FILE")
+pass=$(sed -n '2p' "$CREDS_FILE")
+echo "$(date '+%%Y-%%m-%%d %%H:%%M:%%S') user=$user pass_len=${#pass}" >> "$LOG"
 
 if [ -z "$user" ] || [ -z "$pass" ]; then
   echo "$(date '+%%Y-%%m-%%d %%H:%%M:%%S') DENY empty credentials" >> "$LOG"
@@ -531,7 +540,7 @@ CODE=$(curl -s -o /tmp/ovpn-verify-resp.txt -w "%%{http_code}" \
   2>>"$LOG" || echo "000")
 BODY=$(cat /tmp/ovpn-verify-resp.txt 2>/dev/null)
 
-echo "$(date '+%%Y-%%m-%%d %%H:%%M:%%S') HTTP_CODE=$CODE BODY=$BODY" >> "$LOG"
+echo "$(date '+%%Y-%%m-%%d %%H:%%M:%%S') HTTP=$CODE BODY=$BODY" >> "$LOG"
 
 if [ "$CODE" = "200" ]; then
   echo "$(date '+%%Y-%%m-%%d %%H:%%M:%%S') ALLOW" >> "$LOG"