Enforce unique catalog titles on creation

Added a uniqueness check for catalog titles in the creation flow. The service now prevents creating catalogs with duplicate titles by checking for existing entries before insertion. If a duplicate is detected, the API returns a 409 Conflict response. Updated interfaces and repository to support title-based lookups.

DbFirst.API/Controllers/CatalogsController.cs

@@ -36,6 +36,10 @@ public class CatalogsController : ControllerBase
     public async Task<ActionResult<CatalogReadDto>> Create(CatalogWriteDto dto, CancellationToken cancellationToken)
     {
         var created = await _service.CreateAsync(dto, cancellationToken);
+        if (created == null)
+        {
+            return Conflict();
+        }
         return CreatedAtAction(nameof(GetById), new { id = created.Guid }, created);
     }
 

DbFirst.Application/Catalogs/CatalogService.cs

@@ -27,8 +27,14 @@ public class CatalogService : ICatalogService
         return item == null ? null : _mapper.Map<CatalogReadDto>(item);
     }
 
-    public async Task<CatalogReadDto> CreateAsync(CatalogWriteDto dto, CancellationToken cancellationToken = default)
+    public async Task<CatalogReadDto?> CreateAsync(CatalogWriteDto dto, CancellationToken cancellationToken = default)
     {
+        var existing = await _repository.GetByTitleAsync(dto.CatTitle, cancellationToken);
+        if (existing != null)
+        {
+            return null;
+        }
+
         var entity = _mapper.Map<VwmyCatalog>(dto);
         entity.AddedWho = "system";
         entity.AddedWhen = DateTime.UtcNow;
@@ -49,6 +55,7 @@ public class CatalogService : ICatalogService
 
         var entity = _mapper.Map<VwmyCatalog>(dto);
         entity.Guid = id;
+        entity.CatTitle = existing.CatTitle;
         entity.AddedWho = existing.AddedWho;
         entity.AddedWhen = existing.AddedWhen;
         entity.ChangedWho = "system";

DbFirst.Application/Catalogs/ICatalogService.cs

@@ -4,7 +4,7 @@ public interface ICatalogService
 {
     Task<List<CatalogReadDto>> GetAllAsync(CancellationToken cancellationToken = default);
     Task<CatalogReadDto?> GetByIdAsync(int id, CancellationToken cancellationToken = default);
-    Task<CatalogReadDto> CreateAsync(CatalogWriteDto dto, CancellationToken cancellationToken = default);
+    Task<CatalogReadDto?> CreateAsync(CatalogWriteDto dto, CancellationToken cancellationToken = default);
     Task<CatalogReadDto?> UpdateAsync(int id, CatalogWriteDto dto, CancellationToken cancellationToken = default);
     Task<bool> DeleteAsync(int id, CancellationToken cancellationToken = default);
 }

DbFirst.Domain/Repositories/ICatalogRepository.cs

@@ -6,6 +6,7 @@ public interface ICatalogRepository
 {
     Task<List<VwmyCatalog>> GetAllAsync(CancellationToken cancellationToken = default);
     Task<VwmyCatalog?> GetByIdAsync(int id, CancellationToken cancellationToken = default);
+    Task<VwmyCatalog?> GetByTitleAsync(string title, CancellationToken cancellationToken = default);
     Task<VwmyCatalog> InsertAsync(VwmyCatalog catalog, CancellationToken cancellationToken = default);
     Task<VwmyCatalog?> UpdateAsync(int id, VwmyCatalog catalog, CancellationToken cancellationToken = default);
     Task<bool> DeleteAsync(int id, CancellationToken cancellationToken = default);

DbFirst.Infrastructure/Repositories/CatalogRepository.cs

@@ -25,6 +25,11 @@ public class CatalogRepository : ICatalogRepository
         return await _db.VwmyCatalogs.AsNoTracking().FirstOrDefaultAsync(x => x.Guid == id, cancellationToken);
     }
 
+    public async Task<VwmyCatalog?> GetByTitleAsync(string title, CancellationToken cancellationToken = default)
+    {
+        return await _db.VwmyCatalogs.AsNoTracking().FirstOrDefaultAsync(x => x.CatTitle == title, cancellationToken);
+    }
+
     public async Task<VwmyCatalog> InsertAsync(VwmyCatalog catalog, CancellationToken cancellationToken = default)
     {
         var guidParam = new SqlParameter("@GUID", SqlDbType.Int)