refactor(backend): strengthen domain layer types and add missing page types

- DomainError struct with Code() string method replaces plain errors.New
  sentinels; errors.Is() still works via pointer equality
- UUIDCreatedAt(uuid.UUID) time.Time helper extracts timestamp from UUID v7
- Add TagOffsetPage, CategoryOffsetPage, PoolOffsetPage
- FileListParams fields grouped with comments matching openapi.yaml params
- Fix mismatched comment on UserPage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/domain/category.go

@@ -17,5 +17,13 @@ type Category struct {
 	CreatorID   int16
 	CreatorName string // denormalized
 	IsPublic    bool
-	CreatedAt   time.Time // extracted from UUID v7
+	CreatedAt   time.Time // extracted from UUID v7 via UUIDCreatedAt
+}
+
+// CategoryOffsetPage is an offset-based page of categories.
+type CategoryOffsetPage struct {
+	Items  []Category
+	Total  int
+	Offset int
+	Limit  int
 }