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/postcss/lib/postcss.d.mts

@@ -0,0 +1,69 @@
+export {
+  // Type-only exports
+  AcceptedPlugin,
+
+  AnyNode,
+  atRule,
+  AtRule,
+  AtRuleProps,
+  Builder,
+  ChildNode,
+  ChildProps,
+  comment,
+  Comment,
+  CommentProps,
+  Container,
+  ContainerProps,
+  CssSyntaxError,
+  decl,
+  Declaration,
+  DeclarationProps,
+  // postcss function / namespace
+  default,
+  document,
+  Document,
+  DocumentProps,
+  FilePosition,
+  fromJSON,
+  Helpers,
+  Input,
+
+  JSONHydrator,
+  // This is a class, but it’s not re-exported. That’s why it’s exported as type-only here.
+  type LazyResult,
+  list,
+  Message,
+  Node,
+  NodeErrorOptions,
+  NodeProps,
+  OldPlugin,
+  parse,
+  Parser,
+  // @ts-expect-error This value exists, but it’s untyped.
+  plugin,
+  Plugin,
+  PluginCreator,
+  Position,
+  Postcss,
+  ProcessOptions,
+  Processor,
+  Result,
+  root,
+  Root,
+  RootProps,
+  rule,
+  Rule,
+  RuleProps,
+  Source,
+  SourceMap,
+  SourceMapOptions,
+  Stringifier,
+  // Value exports from postcss.mjs
+  stringify,
+  Syntax,
+  TransformCallback,
+  Transformer,
+  Warning,
+
+  WarningOptions
+} from './postcss.js'