Add `SubscriptionsController`

app/controllers/subscriptions_controller.rb

@@ -0,0 +1,39 @@
+class SubscriptionsController < ApplicationController
+  include TurboStreamable
+
+  before_action :authenticate_user!
+
+  turbo_stream_actions :create, :destroy
+
+  def create
+    answer = Answer.find(params[:answer])
+    result = Subscription.subscribe(current_user, answer)
+
+    respond_to do |format|
+      format.turbo_stream do
+        render turbo_stream: [
+          turbo_stream.replace("subscription-#{answer.id}", partial: "subscriptions/destroy", locals: { answer: }),
+          render_toast(t(result.present? ? ".success" : ".error"), result.present?)
+        ]
+      end
+
+      format.html { redirect_to answer_path(username: answer.user.screen_name, id: answer.id) }
+    end
+  end
+
+  def destroy
+    answer = Answer.find(params[:answer])
+    result = Subscription.unsubscribe(current_user, answer)
+
+    respond_to do |format|
+      format.turbo_stream do
+        render turbo_stream: [
+          turbo_stream.replace("subscription-#{answer.id}", partial: "subscriptions/create", locals: { answer: }),
+          render_toast(t(result.present? ? ".success" : ".error"), result.present?)
+        ]
+      end
+
+      format.html { redirect_to answer_path(username: answer.user.screen_name, id: answer.id) }
+    end
+  end
+end

app/views/subscriptions/_create.html.haml

@@ -0,0 +1,3 @@
+= button_to subscriptions_path(answer: answer.id), class: "dropdown-item", id: "subscription-#{answer.id}" do
+  %i.fa.fa-fw.fa-bell
+  = t("voc.subscribe")

app/views/subscriptions/_destroy.html.haml

@@ -0,0 +1,3 @@
+= button_to subscriptions_path(answer: answer.id), method: :delete, class: "dropdown-item", id: "subscription-#{answer.id}" do
+  %i.fa.fa-fw.fa-bell-slash
+  = t("voc.unsubscribe")

config/routes.rb

@@ -148,6 +148,8 @@ Rails.application.routes.draw do
   post "/inbox/create", to: "inbox#create", as: :inbox_create
   get "/inbox", to: "inbox#show", as: :inbox
 
+  resource :subscriptions, controller: :subscriptions, only: %i[create destroy]
+
   get "/user/:username", to: "user#show"
   get "/@:username", to: "user#show", as: :user
   get "/@:username/a/:id", to: "answer#show", as: :answer