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/migrations/env.py

@@ -17,6 +17,7 @@ load_dotenv()
 from backend.modules.data_entry.db.models.receipt import Receipt, ReceiptAttachment
 from backend.modules.data_entry.db.models.accounting_entry import AccountingEntry
 from backend.modules.data_entry.db.models.nomenclature import SyncedSupplier, LocalSupplier, SyncedCashRegister
+from backend.modules.data_entry.db.models.ocr_settings import UserOCRPreference, OCRJobMetrics
 
 # this is the Alembic Config object, which provides
 # access to the values within the .ini file in use.