feat(classes): weekly recurring schedules with lazy session materialization

class_schedules rules generate concrete session rows for a rolling 28-day
horizon — no cron; materialize runs idempotently on dashboard and public
reads via a partial unique index on (schedule_id, starts_at). Rule delete
stops future sessions; existing sessions keep bookings and overrides.

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

apps/backend/internal/domain/models.go

@@ -405,6 +405,9 @@ type ClassSession struct {
 	BookedCount  int       `json:"bookedCount"`
 	// WaitlistCount is the number of customers queued for a free seat.
 	WaitlistCount int `json:"waitlistCount"`
+	// ScheduleID links the session back to the recurring rule that generated
+	// it; nil for manually created sessions.
+	ScheduleID *string `json:"scheduleId,omitempty"`
 }
 
 // SessionRosterEntry is one booking on a class session roster. WaitlistPosition
@@ -447,6 +450,28 @@ type CreateClassSessionRequest struct {
 	Capacity   *int    `json:"capacity,omitempty"`
 }
 
+// ClassSchedule is a weekly recurring rule for a class template. Concrete
+// sessions are materialized for a rolling horizon; DayOfWeek follows Go's
+// time.Weekday (Sunday=0) and StartsLocal is an "HH:MM" tenant-local time.
+type ClassSchedule 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"`
+	DayOfWeek     int     `json:"dayOfWeek"`
+	StartsLocal   string  `json:"startsLocal"`
+	Active        bool    `json:"active"`
+}
+
+type CreateClassScheduleRequest struct {
+	TemplateID  string  `json:"templateId" binding:"required"`
+	LocationID  *string `json:"locationId,omitempty"`
+	DayOfWeek   int     `json:"dayOfWeek" binding:"min=0,max=6"`
+	StartsLocal string  `json:"startsLocal" binding:"required"` // "HH:MM"
+}
+
 // ============================================
 // CUSTOMER MODELS
 // ============================================