feat(routes): add product cost lookup (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 1e77d71.

Security Overview

• 🔎 Scanned files: 2 changed file(s)
• 🔗 Scan Link: https://staging.branch.zeropath.com/app/repositories/5eea4b94-6a72-403c-af68-60abe8a1d179?scanId=cea8b87c-f69c-4553-94cd-c1b97a6152c9&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► routes/productCost.ts     Implement product cost lookup endpoint ► server.ts     Register new /rest/products/cost route and wire in productCost handler

[zeropath-ai-staging]

✅ No security or compliance issues detected. Reviewed everything up to 1e77d71.

Security Overview

• 🔎 Scanned files: 2 changed file(s)
• 🔗 Scan Link: https://staging.branch.zeropath.com/app/repositories/76e78572-4c4d-4c18-8702-81b7a5a6273c?scanId=ad1530f9-8c86-4ad3-9dd4-489bcceb3519&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► routes/productCost.ts     Implement product cost lookup endpoint ► server.ts     Register new /rest/products/cost route and wire in productCost handler