Fetch thread summary details and reply count in parallel

[kevinaboos]

💡 What: Parallelized the execution of fetch_latest_thread_reply_event and count_thread_replies in fetch_thread_summary_details.
🎯 Why: To reduce the time it takes to load thread details, especially when the initial thread summary is incomplete or missing.
📊 Impact: Latency for loading thread details when the summary is missing is now max(fetch_latest_reply, count_replies) instead of sum(fetch_latest_reply, count_replies).
🔬 Measurement: Verified that the tokio::join! logic correctly handles the optional return types and updates the state. Running cargo check passes. Existing tests run but have unrelated failures.

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PR created automatically by Jules for task 11209851252719820315 started by @kevinaboos

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