feat: Optimize MySQL backend APIs to improve performance

[taimoor-ahmed-1]

This commit introduces query optimizations to reduce database queries and improve response times:

• Fixed N+1 queries in threads_presentor, get_paginated_user_stats, and other methods using select_related/prefetch_related
• Optimized get_read_states to prefetch data in bulk instead of individual queries
• Optimized get_abuse_flagged_count and get_endorsed with bulk aggregations
• Removed duplicate annotations in handle_threads_query
• Added query optimizations across prepare_thread, validate_thread_and_user, and other methods

Performance impact: Reduced queries from O(n) to O(1)/O(k), eliminated N+1 patterns, improved bulk operations. All changes maintain backward compatibility.

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[taimoor-ahmed-1]

@ormsbee this is the initial work to optimize the mysql backend in forum. I am working on providing improvement stats as POC for this work.