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/postcss/README.md

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+# PostCSS
+
+<img align="right" width="95" height="95"
+     alt="Philosopher’s stone, logo of PostCSS"
+     src="https://postcss.org/logo.svg">
+
+PostCSS is a tool for transforming styles with JS plugins.
+These plugins can lint your CSS, support variables and mixins,
+transpile future CSS syntax, inline images, and more.
+
+PostCSS is used by industry leaders including Wikipedia, Twitter, Alibaba,
+and JetBrains. The [Autoprefixer] and [Stylelint] PostCSS plugins are some of the most popular CSS tools.
+
+---
+
+<img src="https://cdn.evilmartians.com/badges/logo-no-label.svg" alt="" width="22" height="16" />  Built by
+ <b><a href="https://evilmartians.com/devtools?utm_source=postcss&utm_campaign=devtools-button&utm_medium=github">Evil Martians</a></b>, go-to agency for <b>developer tools</b>.
+
+---
+
+[Abstract Syntax Tree]: https://en.wikipedia.org/wiki/Abstract_syntax_tree
+[Evil Martians]:        https://evilmartians.com/?utm_source=postcss
+[Autoprefixer]:         https://github.com/postcss/autoprefixer
+[Stylelint]:            https://stylelint.io/
+[plugins]:              https://github.com/postcss/postcss#plugins
+
+
+## Docs
+Read full docs **[here](https://postcss.org/)**.