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>

admin_api/serializers.py

@@ -5,9 +5,10 @@ User = get_user_model()
 class UserSerializer(serializers.ModelSerializer):
     name = serializers.SerializerMethodField()
     role = serializers.SerializerMethodField()
+    partner = serializers.PrimaryKeyRelatedField(read_only=True)
     class Meta:
         model = User
-        fields = ['id', 'email', 'username', 'name', 'role']
+        fields = ['id', 'email', 'username', 'name', 'role', 'partner']
     def get_name(self, obj):
         return f"{obj.first_name} {obj.last_name}".strip() or obj.username
     def get_role(self, obj):