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/parse/_lib/parsers/TimestampMillisecondsParser.d.cts

@@ -0,0 +1,12 @@
+import { Parser } from "../Parser.js";
+import type { ParseFlags, ParseResult } from "../types.js";
+export declare class TimestampMillisecondsParser extends Parser<number> {
+  priority: number;
+  parse(dateString: string): ParseResult<number>;
+  set<DateType extends Date>(
+    date: DateType,
+    _flags: ParseFlags,
+    value: number,
+  ): [DateType, ParseFlags];
+  incompatibleTokens: "*";
+}