feat: add email ingestion processing service and repository (refs #156)

- EmailIngestionRepository: queue CRUD (insert, update status, get,
  find by email ID), pending vehicle association management, mapRow
  pattern for snake_case -> camelCase conversion
- EmailIngestionService: full processing pipeline with sender validation,
  attachment filtering (PDF/PNG/JPG/JPEG/HEIC, <10MB), dual OCR
  classification (fuel vs maintenance), vehicle association logic
  (single-vehicle auto-associate, multi-vehicle pending), retry handling
  (max 3 attempts), and templated email replies (confirmation, failure,
  pending vehicle)
- Updated controller to delegate async processing to service
- Added receipt_processed/receipt_failed/receipt_pending_vehicle to
  TemplateKey union type

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

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

@@ -1,14 +1,19 @@
 /**
  * @ai-summary Email ingestion feature barrel export
- * @ai-context Exports webhook routes for Resend inbound email processing
+ * @ai-context Exports webhook routes, services, and types for Resend inbound email processing
  */
 
 export { emailIngestionWebhookRoutes } from './api/email-ingestion.routes';
+export { EmailIngestionService } from './domain/email-ingestion.service';
+export { EmailIngestionRepository } from './data/email-ingestion.repository';
 export { ResendInboundClient } from './external/resend-inbound.client';
 export type { ParsedEmailResult, ParsedEmailAttachment } from './external/resend-inbound.client';
 export type {
   EmailIngestionQueueRecord,
   EmailIngestionStatus,
+  EmailProcessingResult,
+  ExtractedReceiptData,
+  PendingVehicleAssociation,
   ResendWebhookEvent,
   ResendWebhookEventData,
 } from './domain/email-ingestion.types';