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/routers/__init__.py

@@ -11,6 +11,8 @@ def create_data_entry_router() -> APIRouter:
     - /receipts - Receipt CRUD and workflow
     - /ocr - OCR processing for receipts
     - /nomenclature - Nomenclature syncing from Oracle
+    - /settings - User settings (OCR preferences)
+    - /metrics - OCR analytics and metrics
 
     Returns:
         APIRouter: Configured router for data entry module
@@ -21,10 +23,13 @@ def create_data_entry_router() -> APIRouter:
     from .receipts import router as receipts_router
     from .ocr import router as ocr_router
     from .nomenclature import router as nomenclature_router
+    from .ocr_settings import router as ocr_settings_router
 
     # Include all sub-routers (no prefix - already prefixed in main.py with /api/data-entry)
     router.include_router(receipts_router, prefix="/receipts", tags=["data-entry-receipts"])
     router.include_router(ocr_router, prefix="/ocr", tags=["data-entry-ocr"])
     router.include_router(nomenclature_router, prefix="/nomenclature", tags=["data-entry-nomenclature"])
+    # OCR settings and metrics (endpoints at /settings/* and /metrics/*)
+    router.include_router(ocr_settings_router, tags=["data-entry-settings"])
 
     return router