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/tsconfig.json

@@ -0,0 +1,12 @@
+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "outDir": "./dist",
+    "rootDir": "./src",
+    "declaration": true,
+    "declarationMap": true,
+    "sourceMap": true
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist"]
+}