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>

.env.example

@@ -1,6 +1,7 @@
-# Database (SQLite by default — works out of the box on any server).
-# For Postgres later, change provider in prisma/schema.prisma and set this to your Postgres URL.
-DATABASE_URL="file:./dev.db"
+# Database — PostgreSQL connection string. prisma/schema.prisma is fixed to the
+# postgresql provider, so this must point at a real Postgres instance (local
+# install, Docker container, or managed service) — a SQLite file path won't work.
+DATABASE_URL="postgresql://user:password@localhost:5432/craft2prints?schema=public"
 
 # Stripe — get these from https://dashboard.stripe.com/apikeys
 STRIPE_SECRET_KEY="sk_live_or_test_..."