Add plan deletion, with a confirmation screen listing affected clubs

Due.plan is PROTECT -- a plan that has ever billed anyone can never
truly be removed, on purpose: amount/period_end/grace_until are
frozen on a Due precisely so a later change can't rewrite what was
actually charged, and losing the plan link off an old Due would do
exactly that to every historical invoice.

"Delete" therefore means one of two things, chosen automatically
(billing/services/plans.py):
- never billed anyone -> the row is removed outright.
- has billing history -> soft-deleted (Plan.deleted_at, is_active
  off): hidden from every picker/listing via the new opt-in
  Plan.objects.visible(), but the row survives so old invoices still
  show what they were billed under.

Either way, every club currently on the plan is unsubscribed outright
-- its Subscription row deleted, not just its plan field cleared.
"No plan" was already a fully-understood state everywhere else in the
app, so this reuses it instead of inventing a new one.

Also handles the easy-to-miss second group: a club on a DIFFERENT
plan, mid-trial, configured to convert to the plan being deleted
(Subscription.post_trial_plan). Left alone that would try to convert
onto a hidden/gone plan later; instead that club's trial is ended now
(both trial fields cleared, per the CheckConstraint requiring them
together) so it needs a new plan picked by hand.

The confirmation screen is a real page, not a modal like every other
billing action -- naming exactly which clubs are affected, in both
groups, and that list can be long.

controlpanel/templates/controlpanel/billing.html

@@ -65,6 +65,7 @@
                                 <td class="flex flex-row gap-2 justify-end">
                                     <button class="btn btn-primary btn-sm btn-outline gap-1" type="button" onclick="document.getElementById('{{ plan.pk|dom_id:"plan_price_modal" }}').showModal()">{% lucide "euro" size=14 %} New price</button>
                                     <button class="btn btn-outline btn-sm gap-1" type="button" onclick="document.getElementById('{{ plan.pk|dom_id:"plan_edit_modal" }}').showModal()">{% lucide "pencil" size=14 %} Edit</button>
+                                    <a class="btn btn-error btn-outline btn-sm gap-1" href="{% url 'controlpanel:plan_delete' plan.pk %}">{% lucide "trash-2" size=14 %} Delete</a>
                                 </td>
                             </tr>
                         {% empty %}

controlpanel/templates/controlpanel/plan_delete.html

@@ -0,0 +1,82 @@
+{% extends "controlpanel/base.html" %}
+{% load i18n lucide %}
+
+{% comment %}
+    Confirm-then-delete for a plan. A real page rather than a modal (unlike every other
+    billing action) because the whole point is naming exactly which clubs are affected, and
+    that list can be long. See billing.services.plans for what "delete" actually does.
+{% endcomment %}
+
+{% block heading %}{% blocktrans with plan=plan.name %}Delete “{{ plan }}”{% endblocktrans %}{% endblock heading %}
+
+{% block actions %}
+    <a class="btn btn-outline gap-2" href="{% url 'controlpanel:billing' %}">{% lucide "arrow-left" size=16 %} {% trans "Back to billing" %}</a>
+{% endblock actions %}
+
+{% block panel %}
+    <div class="card mb-6 bg-base-100 shadow">
+        <div class="card-body">
+            <h2 class="card-title text-base">{% lucide "triangle-alert" size=18 %} {% trans "This can't be undone" %}</h2>
+            {% if impact.will_hard_delete %}
+                <p class="text-sm opacity-70">{% blocktrans with plan=plan.name %}“{{ plan }}” has never billed anyone, so it will be removed completely.{% endblocktrans %}</p>
+            {% else %}
+                <p class="text-sm opacity-70">{% blocktrans with plan=plan.name %}“{{ plan }}” has billing history, so it will be hidden rather than removed — past invoices will still show what they were billed under.{% endblocktrans %}</p>
+            {% endif %}
+        </div>
+    </div>
+
+    {% if impact.unsubscribed_clubs %}
+        <div class="card mb-6 bg-base-100 shadow border-l-4 border-error">
+            <div class="card-body">
+                <h2 class="card-title text-base">
+                    {% lucide "building-2" size=18 %}
+                    {% blocktrans count counter=impact.unsubscribed_clubs|length %}{{ counter }} club is currently on this plan{% plural %}{{ counter }} clubs are currently on this plan{% endblocktrans %}
+                </h2>
+                <p class="text-sm opacity-70">{% trans "Deleting this plan removes their subscription outright — each shows as not billed for anything afterwards, the same as a club that was never put on a plan." %}</p>
+                <ul class="mt-2 divide-y divide-base-200">
+                    {% for club in impact.unsubscribed_clubs %}
+                        <li class="py-2 flex items-center justify-between">
+                            <a class="link link-hover font-medium" href="{% url 'controlpanel:club_detail' club.pk %}">{{ club.name }}</a>
+                            <span class="badge badge-error badge-outline gap-1">{% lucide "circle-x" size=12 %} {% trans "Loses this plan" %}</span>
+                        </li>
+                    {% endfor %}
+                </ul>
+            </div>
+        </div>
+    {% endif %}
+
+    {% if impact.broken_trial_clubs %}
+        <div class="card mb-6 bg-base-100 shadow border-l-4 border-warning">
+            <div class="card-body">
+                <h2 class="card-title text-base">
+                    {% lucide "hourglass" size=18 %}
+                    {% blocktrans count counter=impact.broken_trial_clubs|length %}{{ counter }} club's trial is scheduled to switch to this plan{% plural %}{{ counter }} clubs' trials are scheduled to switch to this plan{% endblocktrans %}
+                </h2>
+                <p class="text-sm opacity-70">{% trans "They stay on their current trial plan, but the scheduled switch is cancelled — pick a new plan for them before the trial ends." %}</p>
+                <ul class="mt-2 divide-y divide-base-200">
+                    {% for club in impact.broken_trial_clubs %}
+                        <li class="py-2 flex items-center justify-between">
+                            <a class="link link-hover font-medium" href="{% url 'controlpanel:club_detail' club.pk %}">{{ club.name }}</a>
+                            <span class="badge badge-warning badge-outline gap-1">{% lucide "octagon-alert" size=12 %} {% trans "Trial needs a new plan" %}</span>
+                        </li>
+                    {% endfor %}
+                </ul>
+            </div>
+        </div>
+    {% endif %}
+
+    {% if not impact.has_impact %}
+        <div class="alert alert-info mb-6">
+            {% lucide "info" size=20 %}
+            <span>{% trans "No club is currently on this plan, or has a trial scheduled to switch to it." %}</span>
+        </div>
+    {% endif %}
+
+    <form method="post" action="{% url 'controlpanel:plan_delete' plan.pk %}">
+        {% csrf_token %}
+        <div class="flex flex-row justify-end gap-2">
+            <a class="btn btn-outline gap-2" href="{% url 'controlpanel:billing' %}">{% lucide "x" size=16 %} {% trans "Cancel" %}</a>
+            <button class="btn btn-error gap-2" type="submit">{% lucide "trash-2" size=16 %} {% trans "Delete plan" %}</button>
+        </div>
+    </form>
+{% endblock panel %}