feat: add receipt OCR pipeline (refs #69)

Implement receipt-specific OCR extraction for fuel receipts:

- Pattern matching modules for date, currency, and fuel data extraction
- Receipt-optimized image preprocessing for thermal receipts
- POST /extract/receipt endpoint with field extraction
- Confidence scoring per extracted field
- Cross-validation of fuel receipt data
- Unit tests for all pattern matchers

Extracted fields: merchantName, transactionDate, totalAmount,
fuelQuantity, pricePerUnit, fuelGrade

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ocr/app/preprocessors/__init__.py

@@ -1,10 +1,16 @@
 """Image preprocessors for OCR optimization."""
 from app.services.preprocessor import ImagePreprocessor, preprocessor
 from app.preprocessors.vin_preprocessor import VinPreprocessor, vin_preprocessor
+from app.preprocessors.receipt_preprocessor import (
+    ReceiptPreprocessor,
+    receipt_preprocessor,
+)
 
 __all__ = [
     "ImagePreprocessor",
     "preprocessor",
     "VinPreprocessor",
     "vin_preprocessor",
+    "ReceiptPreprocessor",
+    "receipt_preprocessor",
 ]