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-backend.sh

@@ -59,6 +59,9 @@ start_backend() {
   print_info "Starting unified backend on port $PORT..."
   cd "$BACKEND_DIR"
 
+  # Clear old log file
+  > "$LOG_FILE"
+
   # Activate virtual environment and start uvicorn
   nohup bash -c "source venv/bin/activate && uvicorn main:app --host 0.0.0.0 --port $PORT --reload" > "$LOG_FILE" 2>&1 &