Strengthen ER model integrity

[patrickschiller]

Summary

• model Request.approverId as an Employee relation and restrict hard deletes for employee history rows
• add DB constraints for project/service-order consistency, HH:mm and bitmask validity, schedule defaults, time/absence overlaps, and time ordering
• add API-level overlap/time-window validation plus E2E coverage for the new integrity rules
• fix API E2E Jest resolution for the shared workspace package

Verification

• pnpm exec prisma validate
• pnpm nx run-many -t typecheck
• pnpm nx run-many -t lint (passes with existing warnings)
• pnpm nx run api-e2e:e2e
• pnpm nx run-many -t build