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/login.html

@@ -78,6 +78,15 @@
                 Return to Home
             </a>
         </div>
+
+        {% if allow_signup %}
+        <div class="mt-4 text-center">
+            <span class="text-sm text-gray-600">Don't have an account?</span>
+            <a href="/signup" class="ml-1 text-sm font-medium text-indigo-600 hover:text-indigo-500">
+                Create account
+            </a>
+        </div>
+        {% endif %}
     </main>
     <div class="fixed bottom-4 text-center w-full text-xs text-gray-500">
         DocuElevate {{ app_version|default('', true) }}