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.

docker-compose.yml

@@ -0,0 +1,15 @@
+services:
+  kids-calendar:
+    build: .
+    ports:
+      - "3007:3007"
+    volumes:
+      - kids-calendar-data:/app/data
+    environment:
+      - SESSION_SECRET=${SESSION_SECRET:?set a long random value in .env}
+      - COOKIE_SECURE=${COOKIE_SECURE:-false}
+      - DISABLE_PUBLIC_SIGNUP=${DISABLE_PUBLIC_SIGNUP:-false}
+    restart: unless-stopped
+
+volumes:
+  kids-calendar-data: