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

SKILL.md

@@ -278,10 +278,16 @@ Returns 409 CONFLICT if the slot is no longer available.
 |--------|----------|-------|-------------|
 | GET | `/calendars/{id}/export.ics` | calendars:read | Export calendar as ICS file |
 | POST | `/calendars/import` | calendars:write | Import ICS file (multipart/form-data) |
+| POST | `/calendars/import-url` | calendars:write | Import from external iCal URL |
+| GET | `/cal/{token}/feed.ics` | None (public) | Public iCal feed for subscription |
 
-Export returns `Content-Type: text/calendar`.
+Export returns `Content-Type: text/calendar` with full RFC 5545 support (VALARM, ATTENDEE, all-day events, RRULE).
 
-Import requires multipart form with `calendar_id` (uuid) and `file` (.ics file).
+Import requires multipart form with `calendar_id` (uuid) and `file` (.ics file). Supports VALARM (reminders), ATTENDEE, TZID, VALUE=DATE (all-day).
+
+Import URL accepts JSON body: `{"calendar_id": "uuid", "url": "https://..."}`. Supports http, https, and webcal protocols.
+
+Public feed: Set `is_public` to `true` via `PUT /calendars/{id}` to generate an `ical_url`. This URL can be used to subscribe in Google Calendar, Apple Calendar, Outlook, etc.
 
 ### Users
 
@@ -375,6 +381,14 @@ When `next_cursor` is `null`, there are no more pages. To fetch the next page, p
 
 1. `GET /calendars` or `POST /calendars` - ensure target calendar exists.
 2. `POST /calendars/import` - upload `.ics` file as multipart form data with `calendar_id`.
+   Or: `POST /calendars/import-url` - import from an iCal URL with `{"calendar_id": "...", "url": "https://..."}`.
+
+### Workflow: Make a calendar publicly subscribable
+
+1. `PUT /calendars/{id}` with `{"is_public": true}` - generates a public iCal feed URL.
+2. The response includes `ical_url` (e.g., `https://api.example.com/cal/{token}/feed.ics`).
+3. Share the `ical_url` - anyone can subscribe to it in their calendar app.
+4. To revoke: `PUT /calendars/{id}` with `{"is_public": false}` - removes the feed URL.
 
 ## Important Constraints