feat(bookings): customer self-service rebooking with real slot picker

CustomerBookingView now carries tenantSlug/serviceId/staffId/
classSessionId so the manage page can query live availability; class
bookings resolve their session title as the display label. Reschedule
rejects waitlisted/completed bookings and session-bound class bookings
(400), collapsing a dead branch in the appointment path.

The manage page reschedule dialog replaces free-form date/time inputs
with a date → availability-slot picker filtered to appointment mode,
so customers can only move to a genuinely free slot with the correct
duration; 409 surfaces "slot just taken". Actions are status-gated:
reschedule for confirmed/pending appointments, cancel for live
bookings, explanatory note for class bookings.

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

@@ -490,6 +490,12 @@ type CustomerBookingView struct {
 	CustomerEmail string    `json:"customerEmail"`
 	Service       string    `json:"service"`
 	BusinessName  string    `json:"businessName"`
+	TenantSlug    string    `json:"tenantSlug,omitempty"`
+	ServiceID     *string   `json:"serviceId,omitempty"`
+	StaffID       *string   `json:"staffId,omitempty"`
+	// ClassSessionID identifies session-bound bookings; they cannot be
+	// rescheduled to arbitrary times (see RescheduleBooking).
+	ClassSessionID *string  `json:"classSessionId,omitempty"`
 	StartsAt      time.Time `json:"startsAt"`
 	EndsAt        time.Time `json:"endsAt"`
 	Location      string    `json:"location"`