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>

README.md

@@ -7,7 +7,7 @@ A complete self-hosted e-commerce platform for personalized products (apparel, d
 ### Customer Experience
 - **Design Tool** — In-browser tool to customize templates with text and images
 - **Product Catalog** — Browse and filter by category, color, size, and price
-- **Shopping Cart** — Persistent cart stored in IndexedDB (handles large design files)
+- **Shopping Cart** — Persistent cart stored in IndexedDB
 - **Customer Accounts** — Optional sign-up for order history and faster checkout
 - **Dark Mode** — Full dark mode support with system preference detection
 - **Mobile Responsive** — Works seamlessly on all devices
@@ -42,12 +42,13 @@ A complete self-hosted e-commerce platform for personalized products (apparel, d
 - **Session Expiry** — Auto-logout when browser closes
 
 ### Technical Stack
-- **Frontend** — Next.js 14 (App Router), React, Tailwind CSS
+- **Frontend** — Next.js 16 (App Router, Turbopack), React, Tailwind CSS
 - **Backend** — Next.js server actions, API routes
-- **Database** — SQLite with Prisma ORM
+- **Database** — PostgreSQL with Prisma ORM
+- **File storage** — Uploaded images/documents are compressed with `sharp` and saved as files on disk (`public/uploads/`), not as base64 in the database — see [Image & File Storage](#image--file-storage) below
 - **Payments** — Stripe (production-ready)
 - **Email** — SMTP (Gmail, Outlook, SendGrid, etc.)
-- **Design** — SVG-based mockups, Canvas for rendering
+- **Design** — Fabric.js/Konva canvas for the design tool, SVG-based product mockups
 
 ## Quick Start
 
@@ -80,7 +81,7 @@ A complete self-hosted e-commerce platform for personalized products (apparel, d
 ## Configuration
 
 **Essential (.env):**
-- `DATABASE_URL` — SQLite path (default: `file:./dev.db`)
+- `DATABASE_URL` — PostgreSQL connection string
 - `ADMIN_USER` / `ADMIN_PASSWORD` — Admin login credentials (⚠️ change before deployment)
 - `STRIPE_SECRET_KEY` / `STRIPE_WEBHOOK_SECRET` — Stripe payment keys
 
@@ -103,11 +104,18 @@ A product can appear on both catalogs simultaneously.
 Prices stored in **GBP pence** internally. Customer-facing header currency selector (£/$/€) changes display only — no actual conversion happens until checkout is configured for it.
 
 ### Cart Storage
-Cart persists to **IndexedDB**, not localStorage, because design items carry large image files. IndexedDB has no 5-10MB limit like localStorage.
+Cart persists to **IndexedDB**, not localStorage, since it comfortably holds the design tool's cart items without the ~5-10MB ceiling localStorage imposes.
 
 ### Color Names
 Colors display as friendly names to customers (Navy, Red, Forest Green) instead of hex codes. Admin still sees hex for reference.
 
+### Image & File Storage
+Every uploaded image or document (product photos, gallery photos, custom-request photos, design-approval previews, purchase invoices, expense receipts, images added inside the design tool) is saved as a real file on disk under `public/uploads/<category>/<yyyy>/<mm>/<uuid>.<ext>`, compressed and resized on the way in with `sharp`. Database columns only ever hold the resulting short URL (e.g. `/uploads/gallery/2026/07/<uuid>.jpeg`) — never a base64 data URL. PDFs (invoices/receipts) pass through untouched.
+
+This matters operationally: `public/uploads/` is gitignored but lives permanently on the server's disk, so **backups must cover it alongside the database** (see [SETUP_AND_BUILD.md](./SETUP_AND_BUILD.md)). No web server config is needed to serve these files — Next.js serves everything under `public/` automatically.
+
+See `src/lib/storage.ts` for the upload/compression helper and its per-category presets.
+
 ## Accounting Features
 
 ### Financial Reports