[MEDIUM-2] No Soft-Delete in Prisma Schema

## Finding MEDIUM-2

**Severity**: MEDIUM
**File**: `server/prisma/schema.prisma`

### Description
No deletedAt field in schema. ESLint rule exists but not enforced at DB level.

### Impact
- Data loss on cascade deletes
- No audit trail for deletions

### Remediation
Add deletedAt field to relevant models.