fix(auth): bump better-auth to 1.6.33, node to 22

- CVE-2026-53514 (HIGH): better-auth <1.6.11 vulnerable to
  unverified-email invitation acceptance; pinned 1.6.33 (1.7.x breaks
  the migration introspection + schema check on this setup)
- qs override to >=6.16.0 clears moderate DoS via express
- kysely audit fix pulled 0.29.x requiring node>=22; node:20 is EOL
  anyway - both Dockerfiles now on node:22-alpine, matching CI
- npm audit: 0 vulnerabilities; e2e verified (migrations run,
  sign-up returns session)

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/auth/package.json

@@ -8,10 +8,13 @@
     "start": "node src/server.js"
   },
   "dependencies": {
-    "better-auth": "^1.3.4",
+    "better-auth": "1.6.33",
     "dotenv": "^16.6.1",
     "express": "^4.21.2",
     "kysely": "^0.28.15",
     "pg": "^8.16.3"
+  },
+  "overrides": {
+    "qs": "^6.16.0"
   }
 }