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/extractors/__init__.py

@@ -8,6 +8,10 @@ from app.extractors.receipt_extractor import (
     ExtractedField,
 )
 from app.extractors.fuel_receipt import FuelReceiptExtractor, fuel_receipt_extractor
+from app.extractors.maintenance_receipt_extractor import (
+    MaintenanceReceiptExtractor,
+    maintenance_receipt_extractor,
+)
 from app.extractors.manual_extractor import (
     ManualExtractor,
     manual_extractor,
@@ -27,6 +31,8 @@ __all__ = [
     "ExtractedField",
     "FuelReceiptExtractor",
     "fuel_receipt_extractor",
+    "MaintenanceReceiptExtractor",
+    "maintenance_receipt_extractor",
     "ManualExtractor",
     "manual_extractor",
     "ManualExtractionResult",