feat(availability): per-staff working hours and class/session management

- Working-hours repo accepts nullable staff scope (pgx + memory); staff
  rules live in availability_rules and drive public slot generation
- Protected routes for staff working hours, class templates, and class
  sessions with tenant-ownership checks and input validation
- Sessions derive endsAt/capacity from templates; list resolves location
  names and live booked counts; template delete cascades sessions,
  session delete preserves bookings via SET NULL (mirrored in memory)
- Zones page: staff-scope selector on working hours, new Classes tab
  with template/session CRUD, CS+EN strings, mobile tab wrapping
- Normalize availability_rules times to HH:MM in the service layer

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/domain/models.go

@@ -368,6 +368,45 @@ type UpdateBlockedDayRequest struct {
 	Type   string `json:"type,omitempty"`
 }
 
+// ============================================
+// CLASS MODELS
+// ============================================
+
+type ClassTemplate struct {
+	ID              string    `json:"id"`
+	TenantID        string    `json:"tenantId"`
+	Title           string    `json:"title"`
+	DurationMinutes int       `json:"durationMinutes"`
+	Capacity        int       `json:"capacity"`
+	CreatedAt       time.Time `json:"createdAt"`
+}
+
+type UpsertClassTemplateRequest struct {
+	Title           string `json:"title" binding:"required"`
+	DurationMinutes int    `json:"durationMinutes" binding:"required"`
+	Capacity        int    `json:"capacity" binding:"required"`
+}
+
+type ClassSession struct {
+	ID           string    `json:"id"`
+	TenantID     string    `json:"tenantId"`
+	TemplateID   string    `json:"templateId"`
+	TemplateTitle string   `json:"templateTitle"`
+	LocationID   *string   `json:"locationId,omitempty"`
+	LocationName string    `json:"locationName,omitempty"`
+	StartsAt     time.Time `json:"startsAt"`
+	EndsAt       time.Time `json:"endsAt"`
+	Capacity     int       `json:"capacity"`
+	BookedCount  int       `json:"bookedCount"`
+}
+
+type CreateClassSessionRequest struct {
+	TemplateID string  `json:"templateId" binding:"required"`
+	LocationID *string `json:"locationId,omitempty"`
+	StartsAt   string  `json:"startsAt" binding:"required"` // RFC3339
+	Capacity   *int    `json:"capacity,omitempty"`
+}
+
 // ============================================
 // CUSTOMER MODELS
 // ============================================