feat(ocr): Add docTR OCR engine with metrics infrastructure

Add docTR as primary OCR engine with 2-tier sequential processing,
OCR metrics tracking, and simplified engine selection.

Features:
- docTR OCR engine with light+medium preprocessing tiers
- doctr_plus mode with early exit optimization (~65% fast path)
- OCR metrics dashboard with per-engine statistics
- User OCR preference persistence
- Parallel worker pool for OCR processing
- Cross-validation for extraction quality

Engine options: tesseract, doctr, doctr_plus (recommended), paddleocr

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

start-prod.sh

@@ -151,6 +151,10 @@ else
     source .env
     set +a
 
+    # Clear old log files
+    > /tmp/unified_backend_prod.log
+    > /tmp/unified_frontend_prod.log
+
     # Start backend with auto-restart on crash (OOM protection)
     print_message "Starting unified backend with auto-restart (includes Reports, Data Entry, and Telegram bot)..."
     nohup ./run-with-restart.sh 8000 /tmp/unified_backend_prod.log > /dev/null 2>&1 &