fix: allow partner portal SSR to reach admin_api (/me/) for impersonation

ALLOWED_HOSTS was missing partner.eventifyplus.com + docker internal
hostnames (eventify-backend, eventify-django). Partner Next.js
server-side authorize() fetch to /api/v1/auth/me/ was rejected with
HTTP 400 DisallowedHost, so admin "Login as Partner" redirected to
/login?error=ImpersonationFailed instead of /dashboard.

Also added `partner` FK to UserSerializer so the /me/ response exposes
the partner id the portal needs to set session.user.partnerId.

Deployed to both eventify-backend and eventify-django containers via
docker cp + HUP.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

eventify/settings.py

@@ -18,6 +18,9 @@ ALLOWED_HOSTS = [
     'backend.eventifyplus.com',
     'admin.eventifyplus.com',
     'app.eventifyplus.com',
+    'partner.eventifyplus.com',
+    'eventify-backend',
+    'eventify-django',
     'localhost',
     '127.0.0.1',
 ]