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/@tailwindcss/vite/dist/index.d.mts

@@ -0,0 +1,13 @@
+import { Plugin } from 'vite';
+
+type PluginOptions = {
+    /**
+     * Optimize and minify the output CSS.
+     */
+    optimize?: boolean | {
+        minify?: boolean;
+    };
+};
+declare function tailwindcss(opts?: PluginOptions): Plugin[];
+
+export { type PluginOptions, tailwindcss as default };