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>

.gitignore

@@ -460,6 +460,8 @@ venv.bak/
 venv.bak/
 venv/
 venv/
+venv-win/
+ocr_benchmark_*.json
 wallet/
 wheels/
 wheels/
@@ -520,5 +522,6 @@ backend/data/cache/*.db
 backend/data/receipts/*.db
 backend/data/telegram/*.db
 backend/data/receipts/uploads/*
+backend/data/ocr_queue/
 !backend/data/*/.gitkeep