perf(quick-wins): batch collaborator resolution, debounce SSE refresh, loading states

- Eliminate N+1 on resolveCollaborator: add batchResolveCollaborators() in
  auth.ts (2 DB queries max regardless of session count), update all 4
  workshop services to use post-batch mapping
- Debounce router.refresh() in useLive.ts (300ms) to group simultaneous
  SSE events and avoid cascade re-renders
- Call cleanupOldEvents fire-and-forget in createEvent to purge old SSE
  events inline without blocking the response
- Add loading.tsx skeletons on /sessions and /users matching actual page
  layout (PageHeader + content structure)
- Lazy-load ShareModal via next/dynamic in BaseSessionLiveWrapper to reduce
  initial JS bundle

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/collaboration/BaseSessionLiveWrapper.tsx

@@ -1,10 +1,12 @@
 'use client';
 
 import { useState, useCallback } from 'react';
+import dynamic from 'next/dynamic';
 import { useLive, type LiveEvent } from '@/hooks/useLive';
 import { CollaborationToolbar } from './CollaborationToolbar';
-import { ShareModal } from './ShareModal';
 import type { ShareRole } from '@prisma/client';
+
+const ShareModal = dynamic(() => import('./ShareModal').then((m) => m.ShareModal), { ssr: false });
 import type { TeamWithMembers, Share } from '@/lib/share-utils';
 
 export type LiveApiPath = 'sessions' | 'motivators' | 'weather' | 'year-review' | 'weekly-checkin' | 'gif-mood';