perf: add compound product indexes

[priyansh0602]

📋 What does this PR do?

Adds compound MongoDB indexes to the Product schema to improve frequent product queries that filter by category and sort/filter by price or rating.

Added:

• { category: 1, price: 1 }
• { category: 1, rating: -1 }

Existing indexes were left unchanged.

🔗 Related Issue

Closes #955

🧪 How was this tested?

Ran the server test suite locally with:

npm test

Result:

• 3 test suites passed
• 78 tests passed
• 0 tests failed

📸 Screenshots (if UI changes)

N/A — no UI changes.

✅ Checklist

• I've read the CONTRIBUTING guide
• My code follows the project's style guidelines
• I've tested my changes locally
• I've linked the related issue
• I haven't introduced any new secrets or API keys