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/test.d.cts

@@ -0,0 +1,14 @@
+export declare function assertType<Type>(_value: Type): void;
+export declare namespace assertType {
+  type Equal<T, U> =
+    Exclude<T, U> extends never
+      ? Exclude<U, T> extends never
+        ? true
+        : false
+      : false;
+}
+export declare function resetDefaultOptions(): void;
+export declare function generateOffset(originalDate: Date): string;
+export declare function fakeDate(date: number | Date): {
+  fakeNow: (date: number | Date) => void;
+};