Implement product recommendation endpoint

[Trishanthsai]

Closes #449

Description

What changed

• Implemented the getRelatedProducts endpoint.
• Added a recommendation engine based on historical purchase data using MongoDB aggregation.
• Added product ID validation.
• Added product existence checks.
• Excluded the requested product from recommendations.
• Filtered out soft-deleted products.
• Returned the top related products while preserving recommendation order.
• Added appropriate error handling for invalid IDs, missing products, and empty recommendations.

Implementation

• Used MongoDB aggregation ($match, $unwind, $group, $sort, $limit) to identify products frequently bought together from completed orders.
• Retrieved the corresponding product details and returned them in ranked order.

[vercel]

@Trishanthsai is attempting to deploy a commit to the niharika-mente's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Trishanthsai]

Hi @Aamod007
I've completed the implementation for this issue and opened a PR.
Implemented

• Recommendation endpoint based on historical completed orders.
• Products frequently bought together using MongoDB aggregation.
• Validation for invalid/non-existent product IDs.
• Soft-deleted products are excluded.
• Graceful handling when no recommendations are found.

The repository currently has unrelated frontend/backend CI issues that are outside the scope of this feature, but the recommendation endpoint implementation itself is complete.

Thank you!