feat(auth): add local user signup, email verification, and Stripe billing

- Add LocalUser model with bcrypt password hashing, email verification
  tokens, and password reset tokens
- Add ALLOW_LOCAL_SIGNUP config flag (requires SMTP to be configured)
- Add Stripe billing config fields (STRIPE_SECRET_KEY, etc.)
- Add stripe_customer_id to UserProfile and stripe_price_id_monthly/
  stripe_price_id_yearly to SubscriptionPlan
- Create migration 018_add_local_users_and_billing
- Add app/utils/local_auth.py: hash_password, verify_password,
  generate_token, is_token_expired, send_verification_email,
  send_password_reset_email, build_session_user
- Add app/api/local_auth.py: signup, email verification, password reset
  endpoints plus signup/verify-email-sent/reset-password page routes
- Add app/api/billing.py: Stripe Checkout, Customer Portal, and webhook
  endpoints; syncs subscription tier from webhook events
- Update auth() to check LocalUser table before admin credentials fallback
- Update login() to pass allow_signup context variable to template
- Add signup.html, verify_email_sent.html, password_reset_form.html,
  billing_success.html templates (Alpine.js, Tailwind, WCAG 2.1 AA)
- Update login.html to show 'Create account' link when signup enabled
- Update pricing.html CTA buttons to use Stripe Checkout for paid tiers
- Add docs/BillingSetup.md with setup guide, webhook config, compliance
- Add tests/test_local_auth.py (42 tests) and tests/test_billing.py
  (31 tests); all 106 tests in the modified test suite pass
- Add stripe>=7.0.0,<15.0.0 to requirements.txt

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/templates/pricing.html

@@ -117,6 +117,13 @@
           <a href="/login"
              class="block w-full text-center py-3 px-4 rounded-lg border-2 border-blue-600 text-blue-600 font-semibold hover:bg-blue-50 transition"
           >{{ tier.cta }}</a>
+          {% elif tier.stripe_price_id_monthly or tier.stripe_price_id_yearly %}
+          <button
+             type="button"
+             onclick="startCheckout('{{ tier.id }}')"
+             class="block w-full text-center py-3 px-4 rounded-lg {% if tier.highlight %}bg-indigo-600 text-white font-semibold hover:bg-indigo-700 transition shadow-md{% else %}bg-blue-600 text-white font-semibold hover:bg-blue-700 transition{% endif %}"
+             style="min-height:44px;"
+          >{{ tier.cta }}</button>
           {% elif tier.highlight %}
           <a href="/login"
              class="block w-full text-center py-3 px-4 rounded-lg bg-indigo-600 text-white font-semibold hover:bg-indigo-700 transition shadow-md"
@@ -377,3 +384,41 @@
   </div>
 </div>
 {% endblock %}
+
+{% block extra_scripts %}
+<script>
+/**
+ * Initiate a Stripe Checkout session for the given plan.
+ * Uses the currently-selected billing cycle toggle (monthly/yearly).
+ *
+ * @param {string} planId - The plan identifier (e.g. 'starter', 'professional').
+ */
+async function startCheckout(planId) {
+  const cycleEl = document.querySelector('[data-billing-cycle]');
+  const billingCycle = (cycleEl && cycleEl.dataset.billingCycle) ? cycleEl.dataset.billingCycle : 'monthly';
+
+  try {
+    const resp = await fetch('/api/billing/create-checkout-session', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ plan_id: planId, billing_cycle: billingCycle })
+    });
+    if (resp.status === 401) {
+      window.location.href = '/login?message=Please+sign+in+to+subscribe';
+      return;
+    }
+    if (resp.ok) {
+      const data = await resp.json();
+      if (data.checkout_url) {
+        window.location.href = data.checkout_url;
+        return;
+      }
+    }
+    const data = await resp.json().catch(() => ({}));
+    alert(data.detail || 'Unable to start checkout. Please try again.');
+  } catch (e) {
+    alert('Network error. Please try again.');
+  }
+}
+</script>
+{% endblock %}