refactor: remove GORM FK associations, handle relations in business layer (v0.6.1)

- Remove Role association field from User model
- Remove User association field from Token model
- controller/user.go: query Role separately after loading User
- controller/token.go: query User and Role with separate DB calls
- handler/admin.go: introduce userResp type, build role info manually;
  batch-load roles in AdminListUsers to avoid N+1

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internal/model/token.go

@@ -7,7 +7,6 @@ import "time"
 type Token struct {
 	ID         uint       `gorm:"primarykey" json:"id"`
 	UserID     uint       `gorm:"not null;index" json:"user_id"`
-	User       User       `gorm:"foreignKey:UserID" json:"-"`
 	Name       string     `gorm:"not null" json:"name"`
 	Token      string     `gorm:"uniqueIndex;not null" json:"-"`
 	CreatedAt  time.Time  `json:"created_at"`