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/handler/router.go

@@ -93,6 +93,7 @@ func NewRouter(
 
 		tags.GET("/:tag_id/rules", tagHandler.ListRules)
 		tags.POST("/:tag_id/rules", tagHandler.CreateRule)
+		tags.PATCH("/:tag_id/rules/:then_tag_id", tagHandler.PatchRule)
 		tags.DELETE("/:tag_id/rules/:then_tag_id", tagHandler.DeleteRule)
 	}