feat: Initial OIDC integration with django-allauth

I've added django-allauth and configured it for OIDC authentication.
This included changes to settings, URLs, and login templates to support OIDC.
I verified that the User model and UserSettings creation are compatible.
I also added documentation for OIDC environment variables to README.md.

app/templates/users/login.html

@@ -2,6 +2,7 @@
 {% load i18n %}
 {% load settings %}
 {% load crispy_forms_tags %}
+{% load socialaccount %}
 
 {% block title %}Login{% endblock %}
 
@@ -25,6 +26,26 @@
             <div class="card-body">
               <h1 class="h2 card-title text-center mb-4">Login</h1>
               {% crispy form %}
+
+              <div class="mt-3">
+                <h2>{% translate "Or login with:" %}</h2>
+                {% get_providers as socialaccount_providers %}
+                {% if socialaccount_providers %}
+                    <ul class="socialaccount_providers list-unstyled">
+                        {% for provider in socialaccount_providers %}
+                            {% if provider.id == 'oidc' %}
+                            <li class="mt-2">
+                                <a title="{{provider.name}}" class="btn btn-outline-primary w-100 socialaccount_provider {{provider.id}}" href="{% provider_login_url provider.id process="login" %}">
+                                    Login with {{provider.name}}
+                                </a>
+                            </li>
+                            {% endif %}
+                        {% endfor %}
+                    </ul>
+                {% else %}
+                    <p>{% translate "Social login is not configured." %}</p>
+                {% endif %}
+              </div>
             </div>
           </div>
         </div>