Add Authentik OIDC auth (issue #10) + client portal /my-bookings (issue #13)

- api/auth.js: zero-dep OIDC authorization-code flow with PKCE against
  Authentik; HMAC-signed HttpOnly session cookies (SESSION_SECRET)
- requireAdmin now accepts an OIDC session in the admin group; legacy
  ADMIN_SECRET bearer kept for n8n and scripts
- New client endpoints: GET /api/my-bookings, owner-gated contract download
- Bookings created while signed in are bound to the client's OIDC sub
- Dashboard: "Sign in with SSO" alongside passphrase fallback
- New /my-bookings portal page (nginx route + themed page)
- Fix booking modal: content area now scrolls; nav no longer overlaps the
  calendar on short viewports (was swallowing clicks on date cells)
- 10 new Playwright tests; suite green at 67

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nginx.conf

@@ -65,6 +65,11 @@ http {
             try_files $uri /dashboard.html;
         }
 
+        # Client portal (issue #13)
+        location /my-bookings {
+            try_files $uri /my-bookings.html;
+        }
+
         # SPA fallback
         location / {
             try_files $uri $uri/ /index.html;