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/activity/service.go

@@ -0,0 +1,55 @@
+package activity
+
+import (
+	"context"
+	"errors"
+
+	"bookra/apps/backend/internal/db"
+	"bookra/apps/backend/internal/domain"
+)
+
+var ErrTenantMembership = errors.New("tenant membership not found")
+
+const defaultLimit = 50
+const maxLimit = 100
+
+type Service struct {
+	repo db.Repository
+}
+
+func NewService(repo db.Repository) *Service {
+	return &Service{repo: repo}
+}
+
+// List returns the tenant's recent activity, newest first. The feed is
+// tenant-scoped — every dashboard user sees the same audit trail.
+func (s *Service) List(ctx context.Context, principal domain.Principal, limit int) (domain.ActivityFeedResponse, error) {
+	membership, err := s.repo.GetTenantMembershipBySubject(ctx, principal.Subject)
+	if err != nil {
+		return domain.ActivityFeedResponse{}, ErrTenantMembership
+	}
+	if limit <= 0 || limit > maxLimit {
+		limit = defaultLimit
+	}
+	records, err := s.repo.ListActivityEvents(ctx, membership.Tenant.ID, limit)
+	if err != nil {
+		return domain.ActivityFeedResponse{}, err
+	}
+	events := make([]domain.ActivityEvent, 0, len(records))
+	for _, rec := range records {
+		event := domain.ActivityEvent{
+			ID:         rec.ID,
+			ActorKind:  rec.ActorKind,
+			ActorName:  rec.ActorName,
+			Action:     rec.Action,
+			EntityType: rec.EntityType,
+			Summary:    rec.Summary,
+			CreatedAt:  rec.CreatedAt,
+		}
+		if rec.EntityID != nil {
+			event.EntityID = *rec.EntityID
+		}
+		events = append(events, event)
+	}
+	return domain.ActivityFeedResponse{Events: events}, nil
+}