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>

services/darkwatch/src/index.ts

@@ -3,3 +3,5 @@ export * from "./hibp/HIBPService";
 export * from "./matching/MatchingEngine";
 export * from "./alerts/AlertPipeline";
 export * from "./scanner/ScanService";
+export * from "./scheduler/ScanScheduler";
+export * from "./webhooks/WebhookHandler";