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/tapable/lib/MultiHook.js

@@ -0,0 +1,52 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Tobias Koppers @sokra
+*/
+"use strict";
+
+class MultiHook {
+	constructor(hooks, name = undefined) {
+		this.hooks = hooks;
+		this.name = name;
+	}
+
+	tap(options, fn) {
+		for (const hook of this.hooks) {
+			hook.tap(options, fn);
+		}
+	}
+
+	tapAsync(options, fn) {
+		for (const hook of this.hooks) {
+			hook.tapAsync(options, fn);
+		}
+	}
+
+	tapPromise(options, fn) {
+		for (const hook of this.hooks) {
+			hook.tapPromise(options, fn);
+		}
+	}
+
+	isUsed() {
+		for (const hook of this.hooks) {
+			if (hook.isUsed()) return true;
+		}
+		return false;
+	}
+
+	intercept(interceptor) {
+		for (const hook of this.hooks) {
+			hook.intercept(interceptor);
+		}
+	}
+
+	withOptions(options) {
+		return new MultiHook(
+			this.hooks.map((hook) => hook.withOptions(options)),
+			this.name
+		);
+	}
+}
+
+module.exports = MultiHook;