Add OIDC Back-Channel Logout support

Implement OIDC Back-Channel Logout 1.0 (RFC). When enabled, the IdP can
POST a signed logout_token JWT to invalidate user sessions server-side.

- Add BackchannelLogoutHandler: JWT verification via jose, jti replay
  protection with bounded cache, session destruction by sub or sid
- Add oidcSessionId column to sessions table with index for fast lookups
- Add backchannel logout route (POST /auth/openid/backchannel-logout)
- Notify connected clients via socket to redirect to login page
- Add authOpenIDBackchannelLogoutEnabled toggle in schema-driven settings UI
- Migration v2.34.0 adds oidcSessionId column and index
- Polish settings UI: auto-populate loading state, subfolder dropdown
  options, KeyValueEditor fixes, localized descriptions via descriptionKey,
  duplicate key detection, success/error toasts
- Localize backchannel logout toast (ToastSessionEndedByProvider)
- OidcAuthStrategy tests now use real class via require-cache stubbing

package.json

@@ -48,6 +48,7 @@
     "lru-cache": "^10.0.3",
     "node-unrar-js": "^2.0.2",
     "nodemailer": "^6.9.13",
+    "jose": "^4.15.4",
     "openid-client": "^5.6.1",
     "p-throttle": "^4.1.1",
     "passport": "^0.6.0",