Remove Upsert functionality from Catalog API

Removed Upsert endpoint and related service/repository methods. Deleted ExecuteUpsertAsync helper. Insert now throws NotImplementedException as a placeholder. Refactored Delete method in repository; logic unchanged.

DbFirst.API/Controllers/CatalogsController.cs

@@ -50,17 +50,6 @@ public class CatalogsController : ControllerBase
         return Ok(updated);
     }
 
-    [HttpPut("upsert")]
-    public async Task<ActionResult<CatalogReadDto>> Upsert(CatalogWriteDto dto, CancellationToken cancellationToken)
-    {
-        var result = await _service.UpsertAsync(dto, cancellationToken);
-        if (result == null)
-        {
-            return BadRequest();
-        }
-        return Ok(result);
-    }
-
     [HttpDelete("{id:int}")]
     public async Task<IActionResult> Delete(int id, CancellationToken cancellationToken)
     {