feat: implement iCal calendar feed (GRO-107)

- Add icalToken column to staff table
- Add public calendar feed endpoint GET /api/calendar/:staffId.ics
- Add token management routes POST/DELETE /api/staff/:id/ical-token
- Hand-build iCal output (RFC 5545)
- Update staff factory with icalToken field

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/db/migrations/0016_ical_token.sql

@@ -0,0 +1 @@
+ALTER TABLE staff ADD COLUMN ical_token TEXT UNIQUE;