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>

backend/modules/data_entry/db/crud/__init__.py

@@ -2,9 +2,12 @@
 from .receipt import ReceiptCRUD
 from .attachment import AttachmentCRUD
 from .accounting_entry import AccountingEntryCRUD
+from .ocr_settings import OCRPreferenceCRUD, OCRMetricsCRUD
 
 __all__ = [
     "ReceiptCRUD",
     "AttachmentCRUD",
     "AccountingEntryCRUD",
+    "OCRPreferenceCRUD",
+    "OCRMetricsCRUD",
 ]