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);
     }