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/proofs/new/actions.ts

@@ -3,6 +3,7 @@
 import { redirect } from 'next/navigation';
 import { prisma } from '@/lib/prisma';
 import { sendProofEmail } from '@/lib/mail';
+import { saveUploadedFile } from '@/lib/storage';
 
 export async function createProof(formData: FormData) {
   const productId = String(formData.get('productId') ?? '');
@@ -21,9 +22,7 @@ export async function createProof(formData: FormData) {
   const product = await prisma.product.findUnique({ where: { id: productId } });
   if (!product) throw new Error('Product not found.');
 
-  const bytes = Buffer.from(await file.arrayBuffer());
-  const mimeType = file.type || 'image/png';
-  const imageDataUrl = `data:${mimeType};base64,${bytes.toString('base64')}`;
+  const imageUrl = await saveUploadedFile(file, 'proofs');
 
   const unitPrice = priceOverride && String(priceOverride).trim() !== ''
     ? Math.round(Number(priceOverride) * 100)
@@ -37,7 +36,7 @@ export async function createProof(formData: FormData) {
       color,
       quantity,
       unitPrice,
-      imageDataUrl,
+      imageUrl,
       note: note || null,
     },
   });