feat: Add Stripe Customer Portal for API Key Recovery

- Add POST /v1/billing/portal endpoint for customer portal access
- Add GET /v1/billing/recover endpoint for API key recovery
- Implement getKeyByEmail() and getCustomerIdByEmail() service functions
- Add comprehensive test coverage for new endpoints and services
- Create dedicated recovery page at /recovery.html with forms
- Add 'Lost your API key?' link on landing page near pricing
- Update OpenAPI documentation for new endpoints
- Return masked API keys for security (snap_xxxx...xxxx format)
- Log full keys for manual email sending (email service TBD)
- Include proper error handling and input validation

public/index.html

@@ -593,6 +593,8 @@ screenshot = snap.<span class="fn">capture</span>(
     </div>
     <p style="margin-top:32px;color:var(--text-secondary);font-size:.95rem">
       Want to test first? <a href="#playground" style="font-weight:600">Try the playground</a> — free, instant, no signup.
+      <br>
+      Lost your API key? <a href="/recovery.html" style="font-weight:600;color:var(--primary)">Recover it here</a>
     </p>
   </div>
 </section>