fix: eliminate untyped repository returns and standardize DomainException usage across all handlers
- Create typed DTOs (ListingDetailData, ListingSearchItem, ListingSellerItem) for repository read methods
- Replace all Promise<any> and PaginatedResult<any> with concrete types in repository interface and implementation
- Remove `as any` casts in search params by using Prisma enum types (TransactionType, PropertyType)
- Migrate all 16 handlers from NestJS built-in exceptions to domain exceptions (NotFoundException, ValidationException, etc.)
- Add CONTRIBUTING.md documenting error handling convention
- All 230 tests pass, typecheck clean
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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