Add self-hosted kids calendar app

Express + SQLite (node:sqlite, no native build step) family calendar:
parent accounts with household invites, per-child calendars with
save/duplicate/print, a token-gated read-only kiosk view for tablets,
and polling to keep parent and kiosk views in sync. Defaults to port 3007.

server.js

@@ -0,0 +1,13 @@
+const app = require('./src/app');
+const config = require('./src/config');
+
+if (config.sessionSecret === 'dev-secret-change-me') {
+  console.warn(
+    'WARNING: SESSION_SECRET is not set — using an insecure default. ' +
+    'Set SESSION_SECRET to a long random value before exposing this beyond localhost.'
+  );
+}
+
+app.listen(config.port, () => {
+  console.log(`Kids Calendar listening on http://localhost:${config.port}`);
+});