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.

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

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

apps/backend/internal/db/repository.go

@@ -142,6 +142,10 @@ type Repository interface {
 	ListStaffFeedTokens(ctx context.Context, tenantID string) ([]StaffFeedTokenRecord, error)
 	RotateTenantFeedToken(ctx context.Context, tenantID string) (string, error)
 	RotateStaffFeedToken(ctx context.Context, staffID string) (string, error)
+
+	// Activity feed
+	AppendActivityEvent(ctx context.Context, params ActivityEventParams) error
+	ListActivityEvents(ctx context.Context, tenantID string, limit int) ([]ActivityEventRecord, error)
 }
 
 type TenantRecord struct {
@@ -298,6 +302,30 @@ type StaffFeedTokenRecord struct {
 	FeedToken   string
 }
 
+// ActivityEventRecord / ActivityEventParams back the per-tenant audit feed.
+type ActivityEventRecord struct {
+	ID         string
+	TenantID   string
+	ActorKind  string
+	ActorName  string
+	Action     string
+	EntityType string
+	EntityID   *string
+	Summary    string
+	CreatedAt  time.Time
+}
+
+type ActivityEventParams struct {
+	TenantID    string
+	ActorKind   string
+	ActorSubject string
+	ActorName   string
+	Action      string
+	EntityType  string
+	EntityID    *string
+	Summary     string
+}
+
 type CreateStaffParams struct {
 	TenantID    string
 	LocationID  *string
@@ -819,6 +847,7 @@ type MemoryRepository struct {
 	emailTemplates  []EmailTemplateRecord
 	tenantFeedToken string
 	staffFeedTokens map[string]string
+	activityEvents  []ActivityEventRecord
 }
 
 func NewMemoryRepository() *MemoryRepository {
@@ -945,6 +974,26 @@ func NewMemoryRepository() *MemoryRepository {
 		staffFeedTokens: map[string]string{
 			staffID: "demo-staff-feed-token",
 		},
+		activityEvents: []ActivityEventRecord{
+			{
+				ID: uuid.NewString(), TenantID: tenantID, ActorKind: "staff", ActorName: "Demo Stylist",
+				Action: "booking.cancelled", EntityType: "booking",
+				Summary:   "Carol White — Consultation",
+				CreatedAt: now.Add(-50 * time.Hour),
+			},
+			{
+				ID: uuid.NewString(), TenantID: tenantID, ActorKind: "system",
+				Action: "booking.promoted", EntityType: "booking",
+				Summary:   "Bob Smith — Small group mobility class",
+				CreatedAt: now.Add(-26 * time.Hour),
+			},
+			{
+				ID: uuid.NewString(), TenantID: tenantID, ActorKind: "customer", ActorName: "Alice Johnson",
+				Action: "booking.created", EntityType: "booking",
+				Summary:   "Alice Johnson — Relaxation massage",
+				CreatedAt: now.Add(-3 * time.Hour),
+			},
+		},
 	}
 }
 
@@ -1315,6 +1364,35 @@ func (r *MemoryRepository) RotateStaffFeedToken(_ context.Context, staffID strin
 	return "", pgx.ErrNoRows
 }
 
+func (r *MemoryRepository) AppendActivityEvent(_ context.Context, params ActivityEventParams) error {
+	if params.TenantID != r.tenant.ID {
+		return nil
+	}
+	r.activityEvents = append(r.activityEvents, ActivityEventRecord{
+		ID:         uuid.NewString(),
+		TenantID:   params.TenantID,
+		ActorKind:  params.ActorKind,
+		ActorName:  params.ActorName,
+		Action:     params.Action,
+		EntityType: params.EntityType,
+		EntityID:   params.EntityID,
+		Summary:    params.Summary,
+		CreatedAt:  time.Now().UTC(),
+	})
+	return nil
+}
+
+func (r *MemoryRepository) ListActivityEvents(_ context.Context, tenantID string, limit int) ([]ActivityEventRecord, error) {
+	if tenantID != r.tenant.ID {
+		return nil, nil
+	}
+	var events []ActivityEventRecord
+	for i := len(r.activityEvents) - 1; i >= 0 && len(events) < limit; i-- {
+		events = append(events, r.activityEvents[i])
+	}
+	return events, nil
+}
+
 func (r *MemoryRepository) ListAvailabilityRulesByTenant(_ context.Context, tenantID string) ([]AvailabilityRuleRecord, error) {
 	if tenantID != r.tenant.ID {
 		return nil, nil