Implement proof of payment download routes

Added two download routes for proof of payment files:

1. Combined route: /share/proof-of-payment/combined/[id]/
   - Downloads location-level proof of payment for all utilities
   - Queries utilBillsProofOfPayment from location
   - Optimized projection for efficient data transfer

2. Per-bill route: /share/proof-of-payment/per-bill/[id]/
   - Downloads proof of payment for individual bills
   - Parses composite ID format: locationID-billID
   - Finds specific bill in location's bills array
   - Returns bill.proofOfPayment

Both routes:
- Return PDF files with proper Content-Type and headers
- Handle 404 for missing locations/bills/proofs
- Use Base64 to binary conversion for file delivery
- Include Last-Modified header for caching
- Use optimized database projections

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

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

app/[locale]/share/proof-of-payment/combined/[id]/route.tsx

@@ -0,0 +1,34 @@
+import { getDbClient } from '@/app/lib/dbClient';
+import { BillingLocation } from '@/app/lib/db-types';
+import { notFound } from 'next/navigation';
+
+export async function GET(request: Request, { params:{ id } }: { params: { id:string } }) {
+    const locationID = id;
+
+    const dbClient = await getDbClient();
+    const location = await dbClient.collection<BillingLocation>("lokacije")
+        .findOne({ _id: locationID }, {
+            projection: {
+                utilBillsProofOfPayment: 1,
+            }
+        });
+
+    if(!location?.utilBillsProofOfPayment) {
+        notFound();
+    }
+
+    // Convert fileContentsBase64 from Base64 string to binary
+    const fileContentsBuffer = Buffer.from(location.utilBillsProofOfPayment.fileContentsBase64, 'base64');
+
+    // Convert fileContentsBuffer to format that can be sent to the client
+    const fileContents = new Uint8Array(fileContentsBuffer);
+
+    return new Response(fileContents, {
+        status: 200,
+        headers: {
+            'Content-Type': 'application/pdf',
+            'Content-Disposition': `attachment; filename="${location.utilBillsProofOfPayment.fileName}"`,
+            'Last-Modified': `${location.utilBillsProofOfPayment.fileLastModified}`
+        }
+    });
+}