chore(comments): re-point dangling supabase/migrations breadcrumbs

The Supabase purge deleted the archived supabase/migrations/.archived/
directory, but 12 src/ files still referenced its files by path in
JSDoc / TODO comments. The references were misleading (pointing to
deleted files), so re-point them at the canonical location of those
RPCs in db/migrations/0001_init.sql. No code or behavior changes.

Also fixed a stray 'supabase shim returns empty results' comment in
src/app/admin/page.tsx (the shim was already removed in Step 3 of
the refactor).

src/app/admin/page.tsx

@@ -32,7 +32,7 @@ export default async function AdminPage() {
   // so a transient DB/network failure can't crash the whole admin page.
   let dashboardBrandId: string | null = adminUser ? await getActiveBrandId(adminUser) : null;
   if (!dashboardBrandId && adminUser?.role === "platform_admin") {
-    // Direct pg query (the supabase shim returns empty results).
+    // Direct pg query (the legacy query-builder shim returned empty results).
     // This ensures a platform_admin sees real dashboard stats even on first login
     // before they have chosen an active brand.
     try {