create basic backend with login

main-server/app/controllers/api/sessions_controller.rb

@@ -0,0 +1,38 @@
+class Api::SessionsController < ApplicationController
+  skip_before_action :authenticate_request!, only: [:create]
+
+  def create
+    user = User.authenticate_by(params.permit(:nickname, :password))
+
+    if user
+      token = JwtService.encode(user_id: user.id)
+      render json: {
+        user: user_response(user),
+        token: token
+      }
+    else
+      render json: { error: "Invalid nickname or password" }, status: :unauthorized
+    end
+  end
+
+  def destroy
+    # JWT is stateless; client should discard the token.
+    # For server-side revocation, implement a token blacklist here.
+    render json: { message: "Signed out successfully" }
+  end
+
+  def me
+    render json: { user: user_response(current_user) }
+  end
+
+  private
+
+  def user_response(user)
+    {
+      id: user.id,
+      nickname: user.nickname,
+      avatar_url: user.avatar.attached? ? url_for(user.avatar) : nil,
+      created_at: user.created_at
+    }
+  end
+end