feat: add app user routes (logout, user-info, avatar, tickets)

- Logout: clear device sessions on logout
- User info: return expiry/balance based on billing type
- Avatar upload: support jpg/png/gif/webp, max 2MB
- App tickets: create, reply, close tickets for app users
- Add ClosedAt field to Ticket model

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/model/models.go

@@ -427,6 +427,7 @@ type Ticket struct {
 	Status        string         `gorm:"size:20;default:open" json:"status"`     // open, processing, resolved, closed
 	Priority      string         `gorm:"size:20;default:normal" json:"priority"` // low, normal, high, urgent
 	AssignedTo    *uint          `json:"assigned_to"`                            // 分配给的应用开发者ID或平台管理员ID
+	ClosedAt      *time.Time     `json:"closed_at"`
 	CreatedAt     time.Time      `json:"created_at"`
 	UpdatedAt     time.Time      `json:"updated_at"`
 	DeletedAt     gorm.DeletedAt `gorm:"index" json:"-"`