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

internal/api/openapi/specs/schemas.json

@@ -58,6 +58,7 @@
           "name": { "type": "string", "minLength": 1, "maxLength": 80 },
           "color": { "type": "string", "pattern": "^#[0-9A-Fa-f]{6}$", "example": "#22C55E" },
           "is_public": { "type": "boolean" },
+          "ical_url": { "type": "string", "format": "uri", "description": "Public iCal feed URL (only present when is_public is true)" },
           "role": { "type": "string", "enum": ["owner", "editor", "viewer"], "description": "Current user's role on this calendar" },
           "created_at": { "type": "string", "format": "date-time" },
           "updated_at": { "type": "string", "format": "date-time" }