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>

management/bulk_import.py

@@ -211,8 +211,15 @@ def _parse_family_fields(raw):
     family_role_raw = raw.get("family_role", "").strip()
 
     if not family_group:
+        # family_role=child on its own is the migration case this exists for: a
+        # child the club holds with no parent on file yet. It gets a family of
+        # its own so there is something for a parent to join later -- see
+        # members.services.claims. Any other lone role is still a mistake.
         if family_role_raw:
-            errors.append(_("family_role given without a family_group."))
+            family_role = _match_choice(family_role_raw, FamilyMembership.FamilyRole)
+            if family_role == FamilyMembership.FamilyRole.CHILD:
+                return "", family_role, errors
+            errors.append(_("family_role given without a family_group. Only 'child' is allowed on its own, for a child whose parent will register later."))
         return "", None, errors
 
     if not family_role_raw: