fix: additional HTTP status code and error handling corrections

- createBackupHandler: 200 → 201 Created
- addDownloadHandler: 200 → 201 Created, service errors → 500
- createAssistantSessionHandler: 200 → 201 Created
- writeUserMutationError: default error → 500 (was 400)
- resetUserPasswordHandler: service errors → 500, success → 204 NoContent
- createLibraryHandler: service errors → 500 (was 400)

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

internal/handler/backup.go

@@ -16,7 +16,7 @@ func createBackupHandler(svc *service.Container) gin.HandlerFunc {
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 			return
 		}
-		c.JSON(http.StatusOK, info)
+		c.JSON(http.StatusCreated, info)
 	}
 }