feat(04-02): implement CsvValidationService and TemplateRepository with tests
- CsvValidationService: CsvHelper-based parsing with DetectDelimiter, BOM detection,
per-row validation for BulkMemberRow/BulkSiteRow/FolderStructureRow
- TemplateRepository: atomic JSON write (tmp + File.Move) with SemaphoreSlim,
supports GetAll/GetById/Save/Delete/Rename operations
- CsvValidationServiceTests: 9 passing tests (email validation, delimiter detection,
BOM handling, folder/site/member validation)
- TemplateRepositoryTests: 6 passing tests (round-trip, GetAll, delete, rename,
empty directory, non-existent id)
- All previously-skipped scaffold tests now active and passing (15 total)
f3a1c352c7