feat: add source field with 3 options, fix EventListAPI fallback, add is_eventify_event to API response

- Event.source field updated: eventify, community, partner (radio select in form)
- EventListAPI: fallback to all events when pincode returns < 6
- EventListAPI: include is_eventify_event and source in serializer
- Admin API: add source to list serializer
- Django admin: source in list_display, list_filter, list_editable
- Event form template: proper radio button rendering for source field

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

templates/events/event_form.html

@@ -10,15 +10,28 @@
         {% for field in form %}
         <div class="mb-3">
             {{ field.label_tag }}
+            {% if field.name == 'source' %}
+            <div class="mt-2">
+                {% for radio in field %}
+                <div class="form-check">
+                    {{ radio.tag }}
+                    <label class="form-check-label" for="{{ radio.id_for_label }}">{{ radio.choice_label }}</label>
+                </div>
+                {% endfor %}
+            </div>
+            {% else %}
             {{ field }}
+            {% endif %}
             {% for error in field.errors %}
             <div class="text-danger">{{ error }}</div>
             {% endfor %}
         </div>
         {% endfor %}
 
-        <button class="btn btn-primary">Save</button>
-        <a class="btn btn-secondary" href="{% url 'events:event_list' %}">Cancel</a>
+        <div class="mb-3 mt-4">
+            <button class="btn btn-primary">Save</button>
+            <a class="btn btn-secondary" href="{% url 'events:event_list' %}">Cancel</a>
+        </div>
     </form>
 </div>