Rebuild team/group bulk add as a searchable row formset

The old page gave every eligible member a table row, which a club with a
hundred-plus members can't use, and its search was a GET round-trip that
discarded anything already ticked. Now you add rows: pick a person from a
searchable select, pick a position, fill in the details.

The position decides the role. Position.staff_position already distinguishes
them, so a staff position creates a StaffAssignment and anything else a
TeamMembership -- no separate "player or staff?" control that could disagree
with the position picked. Someone joining as both is two rows. Jersey number
and captaincy exist only on TeamMembership, so a staff row rejects them and the
row script greys them out, keyed off the data-staff marker PositionSelect
stamps on staff options.

All-or-nothing on submit: one bad row re-renders the page with every row still
filled in and the offending field flagged, rather than saving the good rows and
losing the rest -- a partial save costs far more when the rows were typed by
hand. Cross-row checks no single row can see (the same person twice, two rows
claiming one jersey) live on the formset's clean(); per-row checks live on the
row form. Eligibility is still never trusted from the POST.

Captain and alternate captain on one row is refused as self-contradictory, but
how many captains a team may have is deliberately left alone: neither the model
nor the single-add form constrains it, and inventing the rule in one entry path
only would be bypassable by adding players one at a time. A test pins that
absence so it reads as a decision rather than an oversight.

Two implementation notes worth keeping: rows are cloned from the template's
parsed content, not its innerHTML, because assigning "<tr>...</tr>" to a
detached <div> silently drops it; and these tables deliberately skip the usual
overflow-x-auto wrapper, which would make a scroll container that clips the
picker's dropdown for every row but the first couple. Removing a row leaves
TOTAL_FORMS alone -- Django reads a form whose fields are absent from the POST
as an unchanged extra and skips it, which is safe, unlike re-indexing live
inputs.

management/tests.py also carries the setUpTestData rationalisation from the
previous commit, which couldn't be split cleanly from the new bulk-add tests.

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

static/js/searchable-select.js

@@ -11,6 +11,12 @@
  */
 (() => {
     function enhance(select) {
+        // Rows added after page load (see team_bulk_add.html) are enhanced on
+        // demand via window.enhanceSearchableSelect, so a select can be handed
+        // here twice -- without this it would grow a second search input.
+        if (select.dataset.searchableReady === "1") return;
+        select.dataset.searchableReady = "1";
+
         const isMultiple = select.multiple;
         const options = Array.from(select.options).filter((option) => option.value !== "");
 
@@ -153,5 +159,10 @@
         }
     }
 
+    // Exposed for forms that clone new rows in after this initial sweep has run
+    // (the bulk-add pages) -- the sweep below only ever sees what's already in
+    // the DOM, and a <template>'s inert content is deliberately not matched.
+    window.enhanceSearchableSelect = enhance;
+
     document.querySelectorAll("select[data-searchable]").forEach(enhance);
 })();