Add public/private calendars, full iCal support, and iCal URL import

- Public/private: toggle is_public via PUT /calendars/{id}; generate/clear
  public_token and return ical_url when public
- Public feed: GET /cal/{token}/feed.ics (no auth) for subscription in
  Google/Apple/Outlook calendars
- Full iCal export: use golang-ical; VALARM, ATTENDEE, all-day (VALUE=DATE),
  RRULE, DTSTAMP, CREATED, LAST-MODIFIED
- Full iCal import: parse TZID, VALUE=DATE, VALARM, ATTENDEE, RRULE
- Import from URL: POST /calendars/import-url with calendar_id + url
- Migration: unique index on public_token, calendar_subscriptions table
- Config: BASE_URL for ical_url; Calendar model + API: ical_url field
- Docs: OpenAPI, llms.txt, README, SKILL.md, about/overview

Made-with: Cursor

go.mod

@@ -3,6 +3,7 @@ module github.com/calendarapi
 go 1.24.0
 
 require (
+	github.com/arran4/golang-ical v0.3.3
 	github.com/go-chi/chi/v5 v5.2.5
 	github.com/golang-jwt/jwt/v5 v5.3.1
 	github.com/golang-migrate/migrate/v4 v4.19.1