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/errors.go

@@ -1,13 +1,21 @@
 package domain
 
-import "errors"
+// DomainError is a typed domain error with a stable machine-readable code.
+// Handlers map these codes to HTTP status codes.
+type DomainError struct {
+	code    string
+	message string
+}
 
-// Sentinel domain errors. Handlers map these to HTTP status codes.
+func (e *DomainError) Error() string { return e.message }
+func (e *DomainError) Code() string  { return e.code }
+
+// Sentinel domain errors. Use errors.Is(err, domain.ErrNotFound) for matching.
 var (
-	ErrNotFound        = errors.New("not found")
-	ErrForbidden       = errors.New("forbidden")
-	ErrUnauthorized    = errors.New("unauthorized")
-	ErrConflict        = errors.New("conflict")
-	ErrValidation      = errors.New("validation error")
-	ErrUnsupportedMIME = errors.New("unsupported MIME type")
+	ErrNotFound        = &DomainError{"not_found", "not found"}
+	ErrForbidden       = &DomainError{"forbidden", "forbidden"}
+	ErrUnauthorized    = &DomainError{"unauthorized", "unauthorized"}
+	ErrConflict        = &DomainError{"conflict", "conflict"}
+	ErrValidation      = &DomainError{"validation_error", "validation error"}
+	ErrUnsupportedMIME = &DomainError{"unsupported_mime", "unsupported MIME type"}
 )