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>

next.config.mjs

@@ -1,7 +1,7 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
   reactStrictMode: true,
-  serverExternalPackages: ['@prisma/client'],
+  serverExternalPackages: ['@prisma/client', 'sharp'],
   experimental: {
     // Invoice uploads (phone photos/PDFs) and product photo uploads go through
     // server actions — the 1MB default rejects typical phone photos.