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/contact/page.tsx

@@ -10,10 +10,11 @@ const ERROR_MESSAGES: Record<string, string> = {
 export default async function ContactPage({
   searchParams,
 }: {
-  searchParams: { sent?: string; error?: string };
+  searchParams: Promise<{ sent?: string; error?: string }>;
 }) {
+  const params = await searchParams;
   const customer = await getCurrentCustomer();
-  const error = searchParams.error ? (ERROR_MESSAGES[searchParams.error] ?? 'Something went wrong.') : null;
+  const error = params.error ? (ERROR_MESSAGES[params.error] ?? 'Something went wrong.') : null;
 
   return (
     <div className="mx-auto max-w-2xl px-6 py-12">
@@ -24,7 +25,7 @@ export default async function ContactPage({
         back to you.
       </p>
 
-      {searchParams.sent === '1' ? (
+      {params.sent === '1' ? (
         <div className="mt-10 border border-splash-blue bg-splash-blue/5 p-6 text-sm">
           Thanks — we&apos;ve got your message and will be in touch soon.
         </div>