Add parent claims: onboarding a roster of children with no parents on file

The migration path for a club arriving with a list of children from a
federation export and no parent records. Children import without logins, each
into a family of their own -- that shape *is* the "nobody is responsible for
this child" state, so there's no unclaimed flag to drift out of step with
reality, and a family drops off the worklist by itself the moment a parent
joins it. `family_role=child` with a blank `family_group` asks for that; any
other lone role is still a mistake in the file.

Verification is a human decision, deliberately. A parent submits a public form
with the child's name and date of birth as free text -- no search, no
autocomplete, and the same response whether or not the child was found, because
the page needs no login and anything that resolved the child would turn it into
a way to enumerate the club's children. An admin matches it from a queue
against a shortlist that only ever contains children with nobody on file, so
approving can never quietly re-parent a child who already has one.

The alternatives were worse. A claim code needs a delivery channel the club may
not have and is a bearer token besides. Matching on name plus birthday hands out
someone else's child to whoever guesses a birthday. The club is the only party
that actually knows its own families.

That form is also the registration: open self-registration is now closed
(shadowing account_signup rather than removing the route, so the URL name
allauth's templates reverse still resolves). The account is created on
approval, not on submission, so a public form can't fill the user table. An
approved parent lands as a guardian -- login and family link, no membership, no
fee -- gets a password-reset link, and a minimal "my family" page.

One bug worth recording: families_awaiting_a_parent first used
annotate(Count(..., filter=...)) over a queryset already filtered on the same
join, so Django reused that join for the counts and a parent with no
ClubMembership of their own -- exactly what a newly linked guardian is -- went
uncounted, leaving the family unclaimed forever. Exists subqueries avoid it. A
test pins both directions.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

templates/account/signup_closed.html

@@ -0,0 +1,20 @@
+{% extends "_base.html" %}
+{% load i18n lucide %}
+
+{% block head_title %}{% trans "Registration is closed" %}{% endblock head_title %}
+
+{% block main %}
+    <div class="flex justify-center">
+        <div class="card w-full max-w-xl bg-base-100 shadow">
+            <div class="card-body">
+                <h1 class="card-title">{% lucide "lock-keyhole" size=20 %} {% trans "Registration is closed" %}</h1>
+                <p>{% blocktrans %}Accounts here are set up by the club, not by signing up.{% endblocktrans %}</p>
+                <p class="text-sm opacity-70">{% blocktrans %}If your child plays at this club, use the link-my-child form. Otherwise ask the club to add you.{% endblocktrans %}</p>
+                <div class="card-actions pt-2">
+                    <a class="btn btn-primary gap-2" href="{% url 'members:parent_claim' %}">{% lucide "users" size=16 %} {% trans "Link me to my child" %}</a>
+                    <a class="btn btn-outline gap-2" href="{% url 'account_login' %}">{% lucide "log-in" size=16 %} {% trans "Sign in" %}</a>
+                </div>
+            </div>
+        </div>
+    </div>
+{% endblock main %}