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/FractionOfSecondParser.js

@@ -0,0 +1,20 @@
+import { Parser } from "../Parser.js";
+
+import { mapValue, parseNDigits } from "../utils.js";
+
+export class FractionOfSecondParser extends Parser {
+  priority = 30;
+
+  parse(dateString, token) {
+    const valueCallback = (value) =>
+      Math.trunc(value * Math.pow(10, -token.length + 3));
+    return mapValue(parseNDigits(token.length, dateString), valueCallback);
+  }
+
+  set(date, _flags, value) {
+    date.setMilliseconds(value);
+    return date;
+  }
+
+  incompatibleTokens = ["t", "T"];
+}