- Salesforce: add validation/duplicate/access/storage error classes and
restore classification in error handler (400/409/403/503 vs generic 502)
- Freebit: add auth/rate-limit/validation/network error classes and
restore result-code-based classification (215, 381, 382)
- Portal: replace unsafe string→enum casts with typed state variables
- BaseRepository: narrow orderBy from unknown to Record<string, "asc"|"desc">
- WHMCS: narrow WhmcsNotFoundError.providerCode from string to union type
- Remove unused UnitOfWork service from PrismaModule
Add structured error code enums to domain package for WHMCS, Salesforce,
and Freebit providers. Create BaseProviderError and typed error classes
for each provider. Update UnifiedExceptionFilter to handle provider errors.
Migrate all three error handler services from DomainHttpException with
brittle string matching to typed error classes with instanceof checks.
