Add file management API with local storage backend

- Implement FileHandler with CRUD operations for files/directories
- Add FileService with business logic and SHA-256 hashing
- Create LocalStorage backend for filesystem persistence
- Add database repository with pagination and name uniqueness
  constraints
- Configure max upload size in storage settings
- Include comprehensive tests for all layers

internal/model/file.go

@@ -7,12 +7,13 @@ import (
 // File represents a file or directory entry in the virtual filesystem.
 type File struct {
 	ID          string    `gorm:"primaryKey;type:varchar(36)" json:"id"`
-	UserID      string    `gorm:"index;type:varchar(36);not null" json:"user_id"`
-	ParentID    *string   `gorm:"index;type:varchar(36)" json:"parent_id"`
-	Name        string    `gorm:"type:varchar(255);not null" json:"name"`
+	UserID      string    `gorm:"index;uniqueIndex:idx_user_parent_name,priority:1;type:varchar(36);not null" json:"user_id"`
+	ParentID    *string   `gorm:"index;uniqueIndex:idx_user_parent_name,priority:2;type:varchar(36)" json:"parent_id"`
+	Name        string    `gorm:"type:varchar(255);not null;uniqueIndex:idx_user_parent_name,priority:3" json:"name"`
 	Size        int64     `gorm:"default:0" json:"size"`
 	MimeType    string    `gorm:"type:varchar(127)" json:"mime_type"`
 	StoragePath string    `gorm:"type:varchar(512)" json:"storage_path"`
+	Hash        string    `gorm:"type:varchar(64)" json:"-"`
 	IsDir       bool      `gorm:"default:false" json:"is_dir"`
 	CreatedAt   time.Time `json:"created_at"`
 	UpdatedAt   time.Time `json:"updated_at"`