Bug: C++ LSHIndex grows indefinitely without memory pruning / TTL policy

[harsharajkumar-273]

Problem Description

In the C++ standalone implementation, the LSHIndex stores post signatures in an in-memory hash map (std::unordered_map bucket structures). Unlike the TypeScript Redis implementation which has a 30-day TTL, the C++ index grows indefinitely and has no mechanism to prune old posts.

Proposed Fix

Introduce a memory management policy to the C++ LSHIndex:

1. Store a timestamp for each indexed post ID.
2. Implement a background cleaner thread or periodic pruning method (pruneOldPosts) that runs after a threshold number of inserts.
3. Bounded-size sliding window (e.g. remove entries older than 30 days) to keep memory footprint constant.