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

migrations/000002_ical_public.up.sql

@@ -0,0 +1,13 @@
+-- Unique index on public_token for fast lookups of public iCal feeds
+CREATE UNIQUE INDEX idx_calendars_public_token ON calendars (public_token) WHERE public_token IS NOT NULL;
+
+-- Track calendars imported from external iCal URLs
+CREATE TABLE calendar_subscriptions (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    calendar_id UUID NOT NULL REFERENCES calendars(id),
+    source_url TEXT NOT NULL,
+    last_synced_at TIMESTAMPTZ,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_calendar_subscriptions_calendar_id ON calendar_subscriptions (calendar_id);