fix(credits-admin): address macroscope review (toast z-index, activity race, audit index)

- toast: add z-index:50 so grant/adjust success toasts render above the
  detail slide-over (scrim z-30 / panel z-40) instead of beneath it.
- loadActivity: request-generation counter drops stale responses, fixing
  the facet-switch / rapid load-more race (wrong cursor / duplicate rows).
- AdminAudit: add @@index([createdAt, id]) matching the global keyset order
  so /audit/recent stops seq-scanning the whole table as history grows.

Author: fbac

prisma/migrations/20260714213447_admin_audit_created_at_id_index/migration.sql

@@ -0,0 +1,2 @@
+-- CreateIndex
+CREATE INDEX "AdminAudit_createdAt_id_idx" ON "AdminAudit"("createdAt", "id");

prisma/schema.prisma

@@ -499,6 +499,7 @@ model AdminAudit {
   @@unique([accountId, idempotencyKey])
   @@index([accountId, createdAt])
   @@index([actorEmail, createdAt])
+  @@index([createdAt, id])
 }
 
 /// Per-PR agent variant (dev XMTP network only). A named bundle pinning an

src/api/v2/credits-admin/handlers/admin-page/script.ts

@@ -50,6 +50,7 @@ export const clientScript = (): string => `
   // --- stubs completed in later tasks ---
   var activityAction="all";
   var activityCursor=null;
+  var activityGen=0;
   function renderActivityRows(rows, append){
     var tb=document.querySelector("#activity-table tbody");
     var html=rows.map(function(r){
@@ -64,8 +65,10 @@ export const clientScript = (): string => `
   }
   function loadActivity(reset){
     if(reset){ activityCursor=null; }
+    var gen=++activityGen;
     var qs="?action="+encodeURIComponent(activityAction)+(activityCursor?("&cursor="+encodeURIComponent(activityCursor)):"");
     guardFetch("/audit/recent"+qs).then(function(r){ return r.json(); }).then(function(j){
+      if(gen!==activityGen) return;
       renderActivityRows(j.rows||[], !reset);
       activityCursor=j.nextCursor;
       el("load-more").classList.toggle("hidden", !j.nextCursor);

src/api/v2/credits-admin/handlers/admin-page/styles.ts

@@ -44,7 +44,7 @@ export const STYLES = `
   .spark .bar { flex:1 1 0; min-width:3px; min-height:2px; background:var(--brand); border-radius:2px 2px 0 0; }
   .card { background:var(--surface); border-radius:var(--radius); padding:16px; margin-bottom:14px; }
   .k { font-size:12px; color:var(--muted); margin-bottom:4px; }
-  .toast { position:fixed; bottom:24px; right:24px; padding:12px 18px; border-radius:8px; color:#fff; opacity:0; transition:opacity .2s; pointer-events:none; }
+  .toast { position:fixed; bottom:24px; right:24px; padding:12px 18px; border-radius:8px; color:#fff; opacity:0; transition:opacity .2s; pointer-events:none; z-index:50; }
   .toast.show { opacity:1; } .toast-success { background:var(--ok); } .toast-error { background:var(--bad); }
 
   /* Detail slide-over */