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/package.json

@@ -34,19 +34,22 @@
     "get-jwks": "^11.0.3",
     "ioredis": "^5.4.2",
     "js-yaml": "^4.1.0",
+    "mailparser": "^3.9.3",
     "node-cron": "^3.0.3",
     "opossum": "^8.0.0",
     "pg": "^8.13.1",
+    "pino": "^9.6.0",
     "resend": "^3.0.0",
     "stripe": "^20.2.0",
+    "svix": "^1.85.0",
     "tar": "^7.4.3",
-    "pino": "^9.6.0",
     "zod": "^3.24.1"
   },
   "devDependencies": {
     "@eslint/js": "^9.17.0",
     "@types/jest": "^29.5.10",
     "@types/js-yaml": "^4.0.9",
+    "@types/mailparser": "^3.4.6",
     "@types/node": "^22.0.0",
     "@types/node-cron": "^3.0.11",
     "@types/opossum": "^8.0.0",