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/json5/lib/register.js

@@ -0,0 +1,13 @@
+const fs = require('fs')
+const JSON5 = require('./')
+
+// eslint-disable-next-line node/no-deprecated-api
+require.extensions['.json5'] = function (module, filename) {
+    const content = fs.readFileSync(filename, 'utf8')
+    try {
+        module.exports = JSON5.parse(content)
+    } catch (err) {
+        err.message = filename + ': ' + err.message
+        throw err
+    }
+}