Scheduling Phase D: two-way external calendar sync (Google, ICS catalog, CalDAV)

[important-new]

Phase D — two-way external calendar sync — is not yet started. Only the one-way foundation exists:

• one-way busy import in server/lib/calendar/sync-busy.ts
• fire-and-forget push in server/lib/calendar/google-calendar.ts (carries a TODO to persist gcalEventId so remote entries can be updated/deleted)

Scope

• external-id link table (OI ↔ Google event id map)
• event update/delete on change (replace the fire-and-forget push)
• import/sync engine
• assigned-schedule ICS feed catalog (beyond the existing "my busy" feed)
• Apple CalDAV (enum placeholder auth_type ['oauth','caldav'] exists; no impl)
• auto-sync cron in server/scheduled.ts with last_synced_at / last_sync_error