feat(activity): per-tenant audit feed in notification dropdown

activity_events records booking lifecycle transitions with the actor
behind each change — staff (dashboard), customer (self-service), or
system (auto-promotion). The notification dropdown leads with the real
feed instead of derived recent-booking guesses.

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

apps/backend/internal/domain/models.go

@@ -539,6 +539,23 @@ type CancelBookingRequest struct {
 	Reason string `json:"reason,omitempty"`
 }
 
+// ActivityEvent is a per-tenant audit entry: who changed what. ActorKind is
+// staff | customer | system; Action is a dotted verb like booking.created.
+type ActivityEvent struct {
+	ID         string    `json:"id"`
+	ActorKind  string    `json:"actorKind"`
+	ActorName  string    `json:"actorName,omitempty"`
+	Action     string    `json:"action"`
+	EntityType string    `json:"entityType"`
+	EntityID   string    `json:"entityId,omitempty"`
+	Summary    string    `json:"summary"`
+	CreatedAt  time.Time `json:"createdAt"`
+}
+
+type ActivityFeedResponse struct {
+	Events []ActivityEvent `json:"events"`
+}
+
 // ============================================
 // ADMIN MODELS
 // ============================================