Replace single-threaded indexing loop with ThreadPoolExecutor.
Default workers = min(cpu_count*2, 16), tunable via DUPFINDER_WORKERS
env var. Pre-loads all existing DB records in one query instead of
N per-file queries. Progress message shows worker count and live
done/total count. Skipped files bulk-stamped in batches of 500.
On an 8-core machine over NAS: ~4-8x faster indexing phase.
On NVMe: up to 16x faster with 16 workers.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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