feat: add Resend inbound webhook endpoint and client (refs #155)

- ResendInboundClient: webhook signature verification via Svix, email
  fetch/download/parse with mailparser
- POST /api/webhooks/resend/inbound endpoint with rawBody, signature
  verification, idempotency check, queue insertion, async processing
- Config: resend_webhook_secret (optional) in secrets schema
- Route registration in app.ts following Stripe webhook pattern

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

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

@@ -0,0 +1,14 @@
+/**
+ * @ai-summary Email ingestion feature barrel export
+ * @ai-context Exports webhook routes for Resend inbound email processing
+ */
+
+export { emailIngestionWebhookRoutes } from './api/email-ingestion.routes';
+export { ResendInboundClient } from './external/resend-inbound.client';
+export type { ParsedEmailResult, ParsedEmailAttachment } from './external/resend-inbound.client';
+export type {
+  EmailIngestionQueueRecord,
+  EmailIngestionStatus,
+  ResendWebhookEvent,
+  ResendWebhookEventData,
+} from './domain/email-ingestion.types';