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/user.go

@@ -8,7 +8,6 @@ type User struct {
 	Username  string    `gorm:"uniqueIndex;not null" json:"username"`
 	Password  string    `gorm:"not null" json:"-"`
 	RoleID    uint      `gorm:"not null" json:"role_id"`
-	Role      Role      `gorm:"foreignKey:RoleID" json:"role"`
 	Active    bool      `gorm:"default:true" json:"active"`
 	CreatedAt time.Time `json:"created_at"`
 	UpdatedAt time.Time `json:"updated_at"`