feat: add receipt classifier and OCR integration (refs #157)

- New ReceiptClassifier module with keyword-based classification for
  fuel vs maintenance receipts from email text and OCR raw text
- Classifier-first pipeline: classify from email subject/body keywords
  before falling back to OCR-based classification
- Fuel keywords: gas, fuel, gallons, octane, pump, diesel, unleaded,
  shell, chevron, exxon, bp
- Maintenance keywords: oil change, brake, alignment, tire, rotation,
  inspection, labor, parts, service, repair, transmission, coolant
- Confident classification (>= 2 keyword matches) routes to specific
  OCR endpoint; unclassified falls back to both endpoints + rawText
  classification + field-count heuristic

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

backend/src/features/email-ingestion/index.ts

@@ -6,14 +6,17 @@
 export { emailIngestionWebhookRoutes } from './api/email-ingestion.routes';
 export { EmailIngestionService } from './domain/email-ingestion.service';
 export { EmailIngestionRepository } from './data/email-ingestion.repository';
+export { ReceiptClassifier } from './domain/receipt-classifier';
 export { ResendInboundClient } from './external/resend-inbound.client';
 export type { ParsedEmailResult, ParsedEmailAttachment } from './external/resend-inbound.client';
 export type {
+  ClassificationResult,
   EmailIngestionQueueRecord,
   EmailIngestionStatus,
   EmailProcessingResult,
   ExtractedReceiptData,
   PendingVehicleAssociation,
+  ReceiptClassificationType,
   ResendWebhookEvent,
   ResendWebhookEventData,
 } from './domain/email-ingestion.types';