feat(routes): add invoice download by filename (rerun)

[ogulcan-gurcaglar]

Controlled re-run: tree is byte-identical to the pre-deploy probe branch, so discovery prompts hit the AI cache and only the confirmation stage differs.

[zeropath-ai-staging]

✅ No security or compliance issues detected. Reviewed everything up to b7d3334.

Security Overview

• 🔎 Scanned files: 1 changed file(s)
• 🔗 Scan Link: https://staging.branch.zeropath.com/app/repositories/5eea4b94-6a72-403c-af68-60abe8a1d179?scanId=12e7a180-1e92-4a5c-8de7-9b545127ae78&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► routes/invoiceDownload.ts      Add new route to serve generated invoice PDFs from ftp/invoices directory

[zeropath-ai-staging]

✅ No security or compliance issues detected. Reviewed everything up to b7d3334.

Security Overview

• 🔎 Scanned files: 1 changed file(s)
• 🔗 Scan Link: https://staging.branch.zeropath.com/app/repositories/76e78572-4c4d-4c18-8702-81b7a5a6273c?scanId=87d1586f-42a6-4fa6-9bc8-7f388f81eaaa&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► routes/invoiceDownload.ts      Add new route to serve generated invoice PDFs from ftp/invoices directory