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/yallist/iterator.js

@@ -0,0 +1,8 @@
+'use strict'
+module.exports = function (Yallist) {
+  Yallist.prototype[Symbol.iterator] = function* () {
+    for (let walker = this.head; walker; walker = walker.next) {
+      yield walker.value
+    }
+  }
+}