⚡ Bolt: [performance improvement] Reuse http.Client for API requests

[himattm]

💡 What: Replaced the local http.Client instantiation in FetchUsage with a package-level usageHttpClient variable.
🎯 Why: Creating a new http.Client for every request prevents TCP connection reuse (keep-alive) and forces new TLS handshakes, which adds significant latency overhead to API requests.
📊 Impact: Decreases latency of repeated API calls by reusing connections. Saves memory and CPU cycles by avoiding repeated TLS handshakes.
🔬 Measurement: Can be measured by benchmarking the time taken for subsequent FetchUsage calls; subsequent calls should be significantly faster as they avoid connection setup time.

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PR created automatically by Jules for task 1498459950166776743 started by @himattm

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