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/mailers/application_mailer.rb

@@ -0,0 +1,4 @@
+class ApplicationMailer < ActionMailer::Base
+  default from: ENV.fetch("MAILER_FROM", "hello@crimata.com")
+  layout "mailer"
+end

app/mailers/customer_mailer.rb

@@ -0,0 +1,9 @@
+class CustomerMailer < ApplicationMailer
+  def magic_link(customer)
+    @customer = customer
+    @url      = verify_url(token: customer.magic_token)
+    @expires  = "15 minutes"
+
+    mail(to: @customer.email, subject: "Your Crimata sign-in link")
+  end
+end