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>

prisma/schema.prisma

@@ -72,21 +72,6 @@ model Product {
   manualSaleItems   ManualSaleItem[]
   stockLevels       StockLevel[]
   purchaseItems     PurchaseItem[]
-  productImages     ProductImage[]
-}
-
-// Bulk-uploaded product images with auto-detected colors. Supports any product type
-// (tees, hoodies, mugs, etc.). User uploads a folder, colors are extracted, then saved
-// as color-swatch references for the product's color palette.
-model ProductImage {
-  id            String   @id @default(cuid())
-  productId     String
-  product       Product  @relation(fields: [productId], references: [id], onDelete: Cascade)
-  color         String // hex color detected from the image, e.g. "#1EA7E0"
-  imageDataUrl  String // the uploaded image itself as a data URL
-  createdAt     DateTime @default(now())
-
-  @@unique([productId, color])
 }
 
 // Quantity on hand for one product/colour/size combination. Purchases increase
@@ -114,7 +99,7 @@ model Purchase {
   id              String   @id @default(cuid())
   supplierName    String
   purchasedAt     DateTime @default(now()) // invoice date, editable — not entry time
-  invoiceDataUrl  String? // the uploaded invoice itself, as a data URL
+  invoiceUrl      String? // disk path under /uploads/invoices/...
   invoiceFileName String?
   total           Int // pence — sum of line totals, recomputed server-side
   notes           String?
@@ -147,7 +132,7 @@ model Expense {
   description   String
   category      String   @default("Stock purchases") // "Stock purchases", "Postage", others as user enters them
   amount        Int // pence
-  receiptDataUrl String? // uploaded receipt as data URL
+  receiptUrl     String? // disk path under /uploads/receipts/...
   receiptFileName String?
   notes         String?
   purchaseId    String? @unique // null = standalone expense; set = auto-created from a Purchase (one-to-one)
@@ -167,7 +152,7 @@ model DesignProof {
   color         String
   quantity      Int      @default(1)
   unitPrice     Int // cents — defaults to the product's base price if not overridden
-  imageDataUrl  String // the finished design image you uploaded, shown as-is to the customer
+  imageUrl      String // disk path under /uploads/proofs/...
   note          String? // optional message shown alongside the design
   status        String   @default("PENDING") // PENDING | APPROVED
   createdAt     DateTime @default(now())
@@ -333,7 +318,7 @@ model CustomRequest {
   customerName  String
   customerEmail String
   customerPhone String?
-  imageDataUrl  String // the photo they want us to work with
+  imageUrl      String // disk path under /uploads/custom-requests/...
   note          String?
   status        String    @default("NEW") // NEW | DONE
   createdAt     DateTime  @default(now())
@@ -430,7 +415,7 @@ model GalleryCategory {
 
 model GalleryPhoto {
   id           String           @id @default(cuid())
-  imageDataUrl String // base64 data URL
+  imageUrl     String // disk path under /uploads/gallery/...
   caption      String?
   categoryId   String? // nullable — a photo can be uncategorized
   category     GalleryCategory? @relation(fields: [categoryId], references: [id])