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/schema.prisma

@@ -50,6 +50,7 @@ model VIP {
   expectedArrival DateTime? // For self-driving arrivals
   airportPickup   Boolean      @default(false)
   venueTransport  Boolean      @default(false)
+  partySize       Int          @default(1) // Total people: VIP + entourage
   notes           String?      @db.Text
   flights         Flight[]
   createdAt       DateTime     @default(now())
@@ -197,6 +198,11 @@ model ScheduleEvent {
   vehicleId       String?
   vehicle         Vehicle?    @relation(fields: [vehicleId], references: [id], onDelete: SetNull)
 
+  // Master/child event hierarchy (shared activity → transport legs)
+  masterEventId   String?
+  masterEvent     ScheduleEvent?  @relation("EventHierarchy", fields: [masterEventId], references: [id], onDelete: SetNull)
+  childEvents     ScheduleEvent[] @relation("EventHierarchy")
+
   // Metadata
   notes           String?     @db.Text