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/file.go

@@ -29,21 +29,26 @@ type File struct {
 	CreatorName     string // denormalized from core.users
 	IsPublic        bool
 	IsDeleted       bool
-	CreatedAt       time.Time // extracted from UUID v7
+	CreatedAt       time.Time // extracted from UUID v7 via UUIDCreatedAt
 	Tags            []Tag     // loaded with the file
 }
 
 // FileListParams holds all parameters for listing/filtering files.
 type FileListParams struct {
-	Filter    string
-	Sort      string
-	Order     string
+	// Pagination
 	Cursor    string
-	Anchor    *uuid.UUID
 	Direction string // "forward" or "backward"
+	Anchor    *uuid.UUID
 	Limit     int
-	Trash     bool
-	Search    string
+
+	// Sorting
+	Sort  string // "content_datetime" | "created" | "original_name" | "mime"
+	Order string // "asc" | "desc"
+
+	// Filtering
+	Filter string // filter DSL expression
+	Search string // substring match on original_name
+	Trash  bool   // if true, return only soft-deleted files
 }
 
 // FilePage is the result of a cursor-based file listing.
@@ -52,3 +57,11 @@ type FilePage struct {
 	NextCursor *string
 	PrevCursor *string
 }
+
+// UUIDCreatedAt extracts the creation timestamp embedded in a UUID v7.
+// UUID v7 stores Unix milliseconds in the most-significant 48 bits.
+func UUIDCreatedAt(id uuid.UUID) time.Time {
+	ms := int64(id[0])<<40 | int64(id[1])<<32 | int64(id[2])<<24 |
+		int64(id[3])<<16 | int64(id[4])<<8 | int64(id[5])
+	return time.Unix(ms/1000, (ms%1000)*int64(time.Millisecond)).UTC()
+}