feat(conf): mask SMTP/OAuth secrets + leave-blank-to-keep on /conf (v1.17.1)

GET /api/conf no longer returns smtp.pass / oauth.jwtSecret in cleartext
(masked to ********). POST treats a blank or ******** secret submission as
"keep the stored value," so editing the From address or token lifetimes no
longer requires re-entering or leaks the SMTP password / JWT secret. The /conf
form fields carry a leave-unchanged hint. Storage stays in OpenBao at
secret/sso-manager/conf (unchanged); no theta-suite policy change needed.

Co-Authored-By: Claude <noreply@anthropic.com>

nodejs/package.json

@@ -1,6 +1,6 @@
 {
   "name": "t42-sso-manager",
-  "version": "1.17.0",
+  "version": "1.17.1",
   "description": "A very simple LDAP management and SSO system",
   "author": [
     {