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-test.sh

@@ -106,6 +106,11 @@ sleep 2
 # Step 2: Start Unified Backend (8000)
 print_message "2. Starting Unified Backend on port 8000..."
 
+# Clear old log files (always, before any port checks)
+> /tmp/unified_backend_test.log
+> /tmp/unified_frontend_test.log
+print_message "Log files cleared"
+
 if check_port 8000; then
     print_warning "Port 8000 already in use - Unified Backend may be running"
 else