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/database.py

@@ -10,9 +10,10 @@ from backend.modules.data_entry.config import settings
 
 
 # Create async engine
+# Note: echo=False to disable SQL query logging (too verbose)
 engine = create_async_engine(
     settings.database_url,
-    echo=settings.debug,
+    echo=False,
     future=True,
 )