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/node-releases/README.md

@@ -0,0 +1,12 @@
+# Node.js releases data
+
+All data is located in `data` directory.
+
+`data/processed` contains `envs.json` with node.js releases data preprocessed to be used by [Browserslist](https://github.com/ai/browserslist) and other projects. Each version in this file contains only necessary info: version, release date, LTS flag/name, and security flag.
+
+`data/release-schedule` contains `release-schedule.json` with node.js releases date and end of life date.
+
+## Installation
+```bash
+npm install node-releases
+```