fix: add social-preview.png, correct test count, add engines field

- Add social-preview.png to version control (referenced in README but untracked)
- Update README: test count 146+ → 162 across 9 suites
- Add engines.node >=22.0.0 to package.json (required for --experimental-sqlite)

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

README.md

@@ -434,7 +434,7 @@ npm run dev        # Starts server with --watch (auto-reload)
 ### Tests
 
 ```bash
-npm test           # 146+ tests across 9 suites
+npm test           # 162 tests across 9 suites
 ```
 
 Tests use Node.js built-in test runner with `--experimental-sqlite` for in-memory SQLite. No running server required.

package.json

@@ -3,6 +3,9 @@
   "version": "0.5.0",
   "description": "Selbstgehosteter Familienplaner — Kalender, Aufgaben, Einkauf, Essensplan, Budget und mehr. Privat, offen, ohne Abo.",
   "main": "server/index.js",
+  "engines": {
+    "node": ">=22.0.0"
+  },
   "scripts": {
     "start": "node server/index.js",
     "dev": "node --watch server/index.js",