Add OpenAPI docs, frontend, migrations, and API updates

- OpenAPI: add missing endpoints (add-from-url, subscriptions, public availability)
- OpenAPI: CalendarSubscription schema, Subscriptions tag
- Frontend app
- Migrations: count_for_availability, subscriptions_sync, user_preferences, calendar_settings
- Config, rate limit, auth, calendar, booking, ICS, availability, user service updates

Made-with: Cursor

frontend/node_modules/caniuse-lite/dist/lib/supported.js

@@ -0,0 +1,9 @@
+module.exports = {
+  y: 1 << 0,
+  n: 1 << 1,
+  a: 1 << 2,
+  p: 1 << 3,
+  u: 1 << 4,
+  x: 1 << 5,
+  d: 1 << 6
+}