feat: add maintenance receipt extraction pipeline with Gemini + regex (refs #150)

- New MaintenanceReceiptExtractor: Gemini-primary extraction with regex
  cross-validation for dates, amounts, and odometer readings
- New maintenance_receipt_validation.py: cross-validation patterns for
  structured field confidence adjustment
- New POST /extract/maintenance-receipt endpoint reusing
  ReceiptExtractionResponse model
- Per-field confidence scores (0.0-1.0) with Gemini base 0.85,
  boosted/reduced by regex agreement

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ocr/app/patterns/__init__.py

@@ -4,6 +4,10 @@ from app.patterns.currency_patterns import CurrencyPatternMatcher, currency_matc
 from app.patterns.fuel_patterns import FuelPatternMatcher, fuel_matcher
 from app.patterns.maintenance_patterns import MaintenancePatternMatcher, maintenance_matcher
 from app.patterns.service_mapping import ServiceMapper, service_mapper
+from app.patterns.maintenance_receipt_validation import (
+    MaintenanceReceiptValidator,
+    maintenance_receipt_validator,
+)
 
 __all__ = [
     "DatePatternMatcher",
@@ -16,4 +20,6 @@ __all__ = [
     "maintenance_matcher",
     "ServiceMapper",
     "service_mapper",
+    "MaintenanceReceiptValidator",
+    "maintenance_receipt_validator",
 ]