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/@babel/helper-globals/data/builtin-lower.json

@@ -0,0 +1,15 @@
+[
+  "decodeURI",
+  "decodeURIComponent",
+  "encodeURI",
+  "encodeURIComponent",
+  "escape",
+  "eval",
+  "globalThis",
+  "isFinite",
+  "isNaN",
+  "parseFloat",
+  "parseInt",
+  "undefined",
+  "unescape"
+]