feat(oauth): restack OAuth backbone

Restack the OAuth backend from PR #5584 onto current master for PAP-9233, preserving the provider registry, OAuth routes, refresh worker, resolver integration, tests, and docs.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/package.json

@@ -76,6 +76,7 @@
     "pino-pretty": "^13.1.3",
     "sharp": "^0.34.5",
     "ws": "^8.19.0",
+    "yaml": "^2.5.0",
     "zod": "^3.24.2"
   },
   "devDependencies": {