feat: add automatic payment/invoice status sync and invoice view page

Core Plugin Enhancements:
- Add afterChange hook to payments collection to auto-update linked invoice status to 'paid' when payment succeeds
- Add afterChange hook to invoices collection for bidirectional payment-invoice relationship management
- Add invoice status sync when manually marked as paid
- Update plugin config types to support collection extension options

Demo Application Features:
- Add professional invoice view page with print-friendly layout (/invoice/[id])
- Add custom message field to payment creation form
- Add invoice API endpoint to fetch complete invoice data with customer info
- Add payment API endpoint to retrieve payment with invoice relationship
- Update payment success page with "View Invoice" button
- Implement beforeChange hook to copy custom message from payment metadata to invoice
- Remove customer collection dependency - use direct customerInfo fields instead

Documentation:
- Update README with automatic status synchronization section
- Add collection extension examples to demo README
- Document new features: bidirectional relationships, status sync, invoice view

Technical Improvements:
- Fix total calculation in invoice API (use 'amount' field instead of 'total')
- Add proper TypeScript types with CollectionSlug casting
- Implement Next.js 15 async params pattern in API routes
- Add customer name/email/company fields to payment creation form

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

dev/app/api/demo/payment/[id]/route.ts

@@ -0,0 +1,63 @@
+import configPromise from '@payload-config'
+import { getPayload } from 'payload'
+
+export async function GET(request: Request, { params }: { params: Promise<{ id: string }> }) {
+  try {
+    const payload = await getPayload({
+      config: configPromise,
+    })
+
+    const { id: paymentProviderId } = await params
+
+    if (!paymentProviderId) {
+      return Response.json(
+        { success: false, error: 'Payment ID is required' },
+        { status: 400 }
+      )
+    }
+
+    // Find payment by providerId (the test provider uses this format)
+    const payments = await payload.find({
+      collection: 'payments',
+      where: {
+        providerId: {
+          equals: paymentProviderId,
+        },
+      },
+      limit: 1,
+    })
+
+    if (!payments.docs.length) {
+      return Response.json(
+        { success: false, error: 'Payment not found' },
+        { status: 404 }
+      )
+    }
+
+    const payment = payments.docs[0]
+
+    return Response.json({
+      success: true,
+      payment: {
+        id: payment.id,
+        providerId: payment.providerId,
+        amount: payment.amount,
+        currency: payment.currency,
+        status: payment.status,
+        description: payment.description,
+        invoice: payment.invoice,
+        metadata: payment.metadata,
+      },
+    })
+  } catch (error) {
+    // eslint-disable-next-line no-console
+    console.error('Failed to fetch payment:', error)
+    return Response.json(
+      {
+        success: false,
+        error: error instanceof Error ? error.message : 'Failed to fetch payment',
+      },
+      { status: 500 }
+    )
+  }
+}