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/caniuse-lite/data/browsers.js

@@ -0,0 +1 @@
+module.exports={A:"ie",B:"edge",C:"firefox",D:"chrome",E:"safari",F:"opera",G:"ios_saf",H:"op_mini",I:"android",J:"bb",K:"op_mob",L:"and_chr",M:"and_ff",N:"ie_mob",O:"and_uc",P:"samsung",Q:"and_qq",R:"baidu",S:"kaios"};