feat: add party size tracking and master event linking

Add partySize field to VIP model (default 1) to track total people
traveling with each VIP including entourage/handlers/spouses. Vehicle
capacity checks now sum party sizes instead of just counting VIPs.

Add masterEventId self-reference to ScheduleEvent for linking transport
legs to shared itinerary items (events, meetings, meals). When creating
a transport event, users can link it to a shared activity and VIPs
auto-populate from the linked event.

Changes:
- Schema: partySize on VIP, masterEventId on ScheduleEvent
- Backend: party-size-aware capacity checks, master/child event includes
- VIP Form: party size input with helper text
- Event Form: party-size capacity display, master event selector
- Event List: party size in capacity and VIP names, master event badges
- Command Center: all VIP names shown with party size indicators

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/prisma/migrations/20260203100000_add_party_size_and_master_events/migration.sql

@@ -0,0 +1,8 @@
+-- AlterTable
+ALTER TABLE "vips" ADD COLUMN "partySize" INTEGER NOT NULL DEFAULT 1;
+
+-- AlterTable
+ALTER TABLE "schedule_events" ADD COLUMN "masterEventId" TEXT;
+
+-- AddForeignKey
+ALTER TABLE "schedule_events" ADD CONSTRAINT "schedule_events_masterEventId_fkey" FOREIGN KEY ("masterEventId") REFERENCES "schedule_events"("id") ON DELETE SET NULL ON UPDATE CASCADE;