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/date-fns/_lib/normalizeDates.d.cts

@@ -0,0 +1,13 @@
+import type { ContextFn, DateArg } from "../types.js";
+export declare function normalizeDates(
+  context: ContextFn<Date> | undefined,
+  ...dates: [DateArg<Date>, DateArg<Date>, DateArg<Date>]
+): [Date, Date, Date];
+export declare function normalizeDates(
+  context: ContextFn<Date> | undefined,
+  ...dates: [DateArg<Date>, DateArg<Date>]
+): [Date, Date];
+export declare function normalizeDates(
+  context: ContextFn<Date> | undefined,
+  ...dates: Array<DateArg<Date> & {}>
+): Date[];