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.js

@@ -0,0 +1,18 @@
+import { constructFrom } from "../../../constructFrom.js";
+import { Parser } from "../Parser.js";
+
+import { parseAnyDigitsSigned } from "../utils.js";
+
+export class TimestampMillisecondsParser extends Parser {
+  priority = 20;
+
+  parse(dateString) {
+    return parseAnyDigitsSigned(dateString);
+  }
+
+  set(date, _flags, value) {
+    return [constructFrom(date, value), { timestampIsSet: true }];
+  }
+
+  incompatibleTokens = "*";
+}