fix: align Node.js version to >=22 across Dockerfile and package.json

Dockerfile used node:20-slim but the project requires Node >=22
(--experimental-sqlite in tests, CI matrix). package.json had a
duplicate engines block where the second (>=20.0.0) silently
overwrote the correct first one (>=22.0.0).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -36,9 +36,6 @@
     "tsdav": "^2.0.10"
   },
   "license": "MIT",
-  "engines": {
-    "node": ">=20.0.0"
-  },
   "devDependencies": {
     "sharp": "^0.34.5"
   }