feat(auth): OIDC / SSO single sign-on (v0.17.0-beta)

Add OpenID Connect single sign-on so the platform can authenticate against
any standards-compliant provider (Entra ID, Okta, Google Workspace, Keycloak,
Authentik, …) alongside local accounts. Resolves the feature request in #73.

Flow (backend-driven Authorization Code + PKCE):
- GET /api/auth/oidc/login builds the authorize URL and stores the PKCE/nonce
  transaction in Redis (single-use, 10m TTL); the callback validates the ID
  token via openid-client (JWKS signature, iss/aud/exp/nonce), resolves the
  identity to a local user, and mints the platform's normal session JWT —
  handed to the SPA via URL fragment. No sessions/cookies introduced.

Identity mapping (users table gains email/oidc_subject/oidc_issuer/
auth_provider; password_hash now nullable; partial unique indexes):
- First sign-in auto-provisions a user, or links to an existing local account
  when the IdP email is verified and matches.
- Roles derive from a configurable IdP group -> role map with a default
  fallback; existing users are never auto-demoted on a groupless login.

Configuration (admin-only, no env/redeploy needed):
- Stored as an app_settings row with the client secret encrypted at rest
  (utils/crypto); masked in responses and excluded from GET /api/settings.
- New UI: Settings -> SSO / OIDC panel (issuer/client, claim + group->role
  mapping, provisioning toggles, discovery test, copyable redirect URI), a
  login-page SSO button, and an SSO badge in Users & Roles.

Security: local break-glass login preserved; PKCE + single-use state; email
linking requires email_verified; same-site-only post-login redirect.

Adds openid-client@^5 (CommonJS). 17 new unit tests (claim/role mapping,
username derivation, config secret round-trip/masking); 114 backend tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "mikrotik-manager-backend",
-  "version": "0.16.6-beta",
+  "version": "0.16.12-beta",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "mikrotik-manager-backend",
-      "version": "0.16.6-beta",
+      "version": "0.16.12-beta",
       "dependencies": {
         "@influxdata/influxdb-client": "^1.33.2",
         "@influxdata/influxdb-client-apis": "^1.35.0",
@@ -22,6 +22,7 @@
         "jsonwebtoken": "^9.0.2",
         "morgan": "^1.10.0",
         "nodemailer": "^9.0.1",
+        "openid-client": "^5.7.1",
         "otpauth": "^9.5.1",
         "pg": "^8.21.0",
         "qrcode": "^1.5.4",
@@ -5865,6 +5866,15 @@
         "url": "https://github.com/chalk/supports-color?sponsor=1"
       }
     },
+    "node_modules/jose": {
+      "version": "4.15.9",
+      "resolved": "https://registry.npmjs.org/jose/-/jose-4.15.9.tgz",
+      "integrity": "sha512-1vUQX+IdDMVPj4k8kOxgUqlcK518yluMuGZwqlr44FS1ppZB/5GWh4rZG89erpOBOJjU/OBsnCVFfapsRz6nEA==",
+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/sponsors/panva"
+      }
+    },
     "node_modules/js-tokens": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/js-tokens/-/js-tokens-4.0.0.tgz",
@@ -6517,6 +6527,15 @@
         "node": ">=0.10.0"
       }
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+      "resolved": "https://registry.npmjs.org/object-hash/-/object-hash-2.2.0.tgz",
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+      "license": "MIT",
+      "engines": {
+        "node": ">= 6"
+      }
+    },
     "node_modules/object-inspect": {
       "version": "1.13.4",
       "resolved": "https://registry.npmjs.org/object-inspect/-/object-inspect-1.13.4.tgz",
@@ -6529,6 +6548,15 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/oidc-token-hash": {
+      "version": "5.2.0",
+      "resolved": "https://registry.npmjs.org/oidc-token-hash/-/oidc-token-hash-5.2.0.tgz",
+      "integrity": "sha512-6gj2m8cJZ+iSW8bm0FXdGF0YhIQbKrfP4yWTNzxc31U6MOjfEmB1rHvlYvxI1B7t7BCi1F2vYTT6YhtQRG4hxw==",
+      "license": "MIT",
+      "engines": {
+        "node": "^10.13.0 || >=12.0.0"
+      }
+    },
     "node_modules/on-finished": {
       "version": "2.4.1",
       "resolved": "https://registry.npmjs.org/on-finished/-/on-finished-2.4.1.tgz",
@@ -6576,6 +6604,39 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/openid-client": {
+      "version": "5.7.1",
+      "resolved": "https://registry.npmjs.org/openid-client/-/openid-client-5.7.1.tgz",
+      "integrity": "sha512-jDBPgSVfTnkIh71Hg9pRvtJc6wTwqjRkN88+gCFtYWrlP4Yx2Dsrow8uPi3qLr/aeymPF3o2+dS+wOpglK04ew==",
+      "license": "MIT",
+      "dependencies": {
+        "jose": "^4.15.9",
+        "lru-cache": "^6.0.0",
+        "object-hash": "^2.2.0",
+        "oidc-token-hash": "^5.0.3"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/panva"
+      }
+    },
+    "node_modules/openid-client/node_modules/lru-cache": {
+      "version": "6.0.0",
+      "resolved": "https://registry.npmjs.org/lru-cache/-/lru-cache-6.0.0.tgz",
+      "integrity": "sha512-Jo6dJ04CmSjuznwJSS3pUeWmd/H0ffTlkXXgwZi+eq1UCmqQwCh+eLsYOYCwY991i2Fah4h1BEMCx4qThGbsiA==",
+      "license": "ISC",
+      "dependencies": {
+        "yallist": "^4.0.0"
+      },
+      "engines": {
+        "node": ">=10"
+      }
+    },
+    "node_modules/openid-client/node_modules/yallist": {
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+      "license": "ISC"
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       "version": "0.9.4",
       "resolved": "https://registry.npmjs.org/optionator/-/optionator-0.9.4.tgz",