refactor: 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/pool.go

@@ -17,7 +17,7 @@ type Pool struct {
 	CreatorName string // denormalized
 	IsPublic    bool
 	FileCount   int
-	CreatedAt   time.Time // extracted from UUID v7
+	CreatedAt   time.Time // extracted from UUID v7 via UUIDCreatedAt
 }
 
 // PoolFile is a File with its ordering position within a pool.
@@ -31,3 +31,11 @@ type PoolFilePage struct {
 	Items      []PoolFile
 	NextCursor *string
 }
+
+// PoolOffsetPage is an offset-based page of pools.
+type PoolOffsetPage struct {
+	Items  []Pool
+	Total  int
+	Offset int
+	Limit  int
+}