Critical fixes:
- Release mutex before HTTP calls in GetStats() to prevent blocking
- Fix m.cmd race between Stop() and Wait() goroutine by capturing locally
- Add context support to GetStats() for proper request cancellation
High priority fixes:
- Add existence check in collectMinerStats() before operating on miners
- Add mutex-protected httpClient getter/setter for thread-safe test mocking
Changes:
- Miner interface now requires context.Context for GetStats()
- Stats HTTP requests timeout after 5 seconds (was 30s client default)
- All callers updated to pass context (service uses request context)
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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