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/db/src/index.ts

@@ -1,4 +1,7 @@
-import { PrismaClient } from "@prisma/client";
+import { PrismaClient } from '@prisma/client';
+import { FieldEncryptionService } from './services/field-encryption.service';
 
 export const prisma = new PrismaClient();
 export default prisma;
+export { FieldEncryptionService };
+export type { PrismaClient };