Fetches data by key (mock implementation ready for KV, D1, R2, etc.)
{
"key": "myKey",
"found": false
}?key=user123 → Fetch data for key "user123"
?key=config → Fetch data for key "config"
💡 This is a mock implementation. Connect to Cloudflare KV, D1, or R2 by adding bindings to wrangler.jsonc