Add tier-based scan scheduler and webhook triggers (FRE-4498)

- ScanScheduler: tier-based scheduling (BASIC=24h, PLUS=6h, PREMIUM=1h)
- WebhookHandler: HMAC-verified webhook ingestion with SCAN_TRIGGER support
- API routes: /scheduler and /webhooks endpoints under /api/v1/darkwatch
- Jobs: scheduled scan checker + webhook retry processor via BullMQ
- Schema: ScanSchedule, WebhookEvent models; ScanJob.scheduledBy field
- Types: ScheduleStatus, WebhookEventType, WebhookTriggerInput
- Tests: scheduler lifecycle + webhook signature/processing tests

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/shared-billing/package.json

@@ -0,0 +1,23 @@
+{
+  "name": "@shieldai/shared-billing",
+  "version": "1.0.0",
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "scripts": {
+    "build": "tsc",
+    "test": "jest",
+    "lint": "eslint src/"
+  },
+  "dependencies": {
+    "stripe": "^15.0.0",
+    "zod": "^3.22.0",
+    "express": "^4.18.0"
+  },
+  "devDependencies": {
+    "@types/express": "^4.17.0",
+    "typescript": "^5.0.0"
+  },
+  "peerDependencies": {
+    "typescript": "^5.0.0"
+  }
+}