refactor(api): enforce protocol-neutral service boundaries

- refactor: move HTTP status and DTO concerns out of model and service
  layers into API and handler code.
- feat: add admin service boundary and route auth through services
  instead of direct repository access.
- test: add architecture and error tests covering package boundaries,
  redaction, and service behavior.
- docs: record the protocol-neutral boundary decision and update
  architecture and roadmap notes.

internal/service/file.go

@@ -22,22 +22,22 @@ import (
 
 // FileInfo is the public representation of a file or directory.
 type FileInfo struct {
-	ID        string    `json:"id"`
-	UserID    string    `json:"user_id"`
-	ParentID  *string   `json:"parent_id"`
-	Name      string    `json:"name"`
-	Size      int64     `json:"size"`
-	MimeType  string    `json:"mime_type"`
-	IsDir     bool      `json:"is_dir"`
-	Hash      string    `json:"hash,omitempty"`
-	CreatedAt time.Time `json:"created_at"`
-	UpdatedAt time.Time `json:"updated_at"`
+	ID        string
+	UserID    string
+	ParentID  *string
+	Name      string
+	Size      int64
+	MimeType  string
+	IsDir     bool
+	Hash      string
+	CreatedAt time.Time
+	UpdatedAt time.Time
 }
 
 // FileList is a paginated list of files.
 type FileList struct {
-	Files []FileInfo `json:"files"`
-	Total int64      `json:"total"`
+	Files []FileInfo
+	Total int64
 }
 
 // FileService handles file management business logic.