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/semver/range.bnf

@@ -0,0 +1,16 @@
+range-set  ::= range ( logical-or range ) *
+logical-or ::= ( ' ' ) * '||' ( ' ' ) *
+range      ::= hyphen | simple ( ' ' simple ) * | ''
+hyphen     ::= partial ' - ' partial
+simple     ::= primitive | partial | tilde | caret
+primitive  ::= ( '<' | '>' | '>=' | '<=' | '=' ) partial
+partial    ::= xr ( '.' xr ( '.' xr qualifier ? )? )?
+xr         ::= 'x' | 'X' | '*' | nr
+nr         ::= '0' | [1-9] ( [0-9] ) *
+tilde      ::= '~' partial
+caret      ::= '^' partial
+qualifier  ::= ( '-' pre )? ( '+' build )?
+pre        ::= parts
+build      ::= parts
+parts      ::= part ( '.' part ) *
+part       ::= nr | [-0-9A-Za-z]+