feat: add PATCH /tags/{id}/rules/{then_id} to activate/deactivate rules

- openapi.yaml: new PATCH endpoint with is_active body, returns TagRule
- backend/service: SetRuleActive calls repo.SetActive then returns updated rule
- backend/handler: PatchRule validates body and delegates to service
- backend/router: register PATCH /:tag_id/rules/:then_tag_id
- frontend: TagRuleEditor uses PATCH instead of delete+recreate
- mock: handle PATCH /tags/{id}/rules/{then_id}

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/service/tag_service.go

@@ -191,6 +191,23 @@ func (s *TagService) CreateRule(ctx context.Context, whenTagID, thenTagID uuid.U
 	})
 }
 
+// SetRuleActive toggles a rule's is_active flag and returns the updated rule.
+func (s *TagService) SetRuleActive(ctx context.Context, whenTagID, thenTagID uuid.UUID, active bool) (*domain.TagRule, error) {
+	if err := s.rules.SetActive(ctx, whenTagID, thenTagID, active); err != nil {
+		return nil, err
+	}
+	rules, err := s.rules.ListByTag(ctx, whenTagID)
+	if err != nil {
+		return nil, err
+	}
+	for _, r := range rules {
+		if r.ThenTagID == thenTagID {
+			return &r, nil
+		}
+	}
+	return nil, domain.ErrNotFound
+}
+
 // DeleteRule removes a tag rule.
 func (s *TagService) DeleteRule(ctx context.Context, whenTagID, thenTagID uuid.UUID) error {
 	return s.rules.Delete(ctx, whenTagID, thenTagID)