Add passwordless email auth with pre-checkout signup

Flow: enter email → magic link → verified → Stripe checkout → dashboard

- SessionsController: new/create (send magic link), sent, verify, destroy
- Customer model: generate_magic_token!, magic_token_valid?, verify_email!
- Migration: magic_token, magic_token_expires_at, email_verified_at + unique indexes
- CustomerMailer + mailer layout with magic link email (html + text)
- CheckoutController: GET /checkout/start requires auth, passes customer_id
  as Stripe metadata; webhook finds customer by metadata and updates record
- DashboardController: requires auth, uses current_customer from session
- ApplicationController: current_customer, require_auth, redirect_after_auth
  (stores return_to so verify sends user back to where they were headed)
- Resend gem + initializer; production uses :resend delivery method
- Dev logs magic link URL to Rails logger instead of sending email
- Pricing page: simple link to /checkout/start (no more JS fetch)
- Layout: Sign in / Dashboard / Sign out nav links

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

app/controllers/dashboard_controller.rb

@@ -1,14 +1,7 @@
 class DashboardController < ApplicationController
+  before_action :require_auth
+
   def show
-    if params[:session_id].present?
-      stripe_session = Stripe::Checkout::Session.retrieve(params[:session_id])
-      @customer = Customer.find_by(stripe_customer_id: stripe_session.customer)
-    elsif params[:email].present?
-      @customer = Customer.find_by(email: params[:email])
-    else
-      redirect_to root_path
-    end
-  rescue Stripe::StripeError
-    redirect_to root_path
+    @customer = current_customer
   end
 end