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/src/vips/dto/create-vip.dto.ts

@@ -4,6 +4,8 @@ import {
   IsOptional,
   IsBoolean,
   IsDateString,
+  IsInt,
+  Min,
 } from 'class-validator';
 import { Department, ArrivalMode } from '@prisma/client';
 
@@ -33,6 +35,11 @@ export class CreateVipDto {
   @IsOptional()
   venueTransport?: boolean;
 
+  @IsInt()
+  @IsOptional()
+  @Min(1)
+  partySize?: number;
+
   @IsString()
   @IsOptional()
   notes?: string;