Move uploaded images to disk storage, fix Next 16 async params/searchParams

Homepage was rendering 71MB of HTML because every uploaded image (gallery,
products, proofs, custom requests, design previews, invoices, receipts) was
stored as base64 in Postgres and double-embedded via RSC hydration payloads.
Adds src/lib/storage.ts (sharp-based compression, disk storage under
public/uploads/) and a new /api/design-uploads endpoint for images added
inside the design tool, migrates existing rows, and drops the now-unused
base64 columns and the dead ProductImage table.

Also fixes a systemic bug from the Next.js 16 upgrade where dynamic pages
across the app still destructured params/searchParams synchronously instead
of awaiting them as Promises, which was silently breaking filters/params and
crashing /products/[slug] outright.

Updates README.md and SETUP_AND_BUILD.md to reflect Postgres + Next.js 16 and
document the new image storage architecture and backup requirements.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/admin/accounts/reconciliation/page.tsx

@@ -9,14 +9,14 @@ import ReconciliationClient from './client';
 export default async function ReconciliationPage({
   searchParams,
 }: {
-  searchParams: { id?: string };
+  searchParams: Promise<{ id?: string }>;
 }) {
   const isAdmin = await isAdminAuthenticated();
   if (!isAdmin) {
     return <div className="px-6 py-12 text-center">Unauthorized</div>;
   }
 
-  const statementId = searchParams.id;
+  const { id: statementId } = await searchParams;
 
   if (!statementId) {
     // Show list of statements