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/app/sessions/loading.tsx

@@ -0,0 +1,32 @@
+export default function SessionsLoading() {
+  return (
+    <main className="mx-auto max-w-7xl px-4">
+      {/* PageHeader skeleton */}
+      <div className="py-6 flex items-start justify-between gap-4">
+        <div className="flex items-center gap-3">
+          <div className="h-10 w-10 bg-card rounded-xl animate-pulse" />
+          <div className="space-y-2">
+            <div className="h-7 w-40 bg-card rounded animate-pulse" />
+            <div className="h-4 w-64 bg-card rounded animate-pulse" />
+          </div>
+        </div>
+        <div className="h-9 w-36 bg-card rounded-lg animate-pulse" />
+      </div>
+
+      <div className="space-y-6">
+        {/* Tabs skeleton */}
+        <div className="flex gap-2 pb-2">
+          {[120, 100, 110, 90, 105].map((w, i) => (
+            <div key={i} className="h-9 bg-card animate-pulse rounded-full" style={{ width: w }} />
+          ))}
+        </div>
+        {/* Cards grid skeleton */}
+        <div className="grid gap-4 md:grid-cols-2 lg:grid-cols-3">
+          {Array.from({ length: 6 }).map((_, i) => (
+            <div key={i} className="h-44 bg-card animate-pulse rounded-xl border border-border" />
+          ))}
+        </div>
+      </div>
+    </main>
+  );
+}