feat(templates): add pagination with total counts to catalog and campaign picker

Test Catalog's pagination showed only 'Page N' with no way to know
how many pages existed, and the campaign 'Add Test' modal's template
picker had no pagination at all — with 2800+ templates in the
catalog, its flat 100-row fetch made most templates unreachable
unless a search happened to match. Adds GET /test-templates/count
(open to any authenticated user, mirrors the list endpoint's filters)
and wires real 'Page N of M' + total-available counts into both.

backend/tests/test_template_count_endpoint.py

@@ -0,0 +1,68 @@
+"""GET /test-templates/count — total matching the same filters as the list
+endpoint, so the catalog UI can compute a true page count."""
+
+import pytest
+
+
+@pytest.fixture
+def technique(api, auth_headers):
+    resp = api(
+        "post", "/api/v1/techniques", auth_headers,
+        json={"mitre_id": "T1059.300", "name": "Count Endpoint Technique"},
+    )
+    assert resp.status_code == 201, resp.text
+    return resp.json()["id"]
+
+
+def _create_template(api, headers, mitre_technique_id, name):
+    resp = api(
+        "post", "/api/v1/test-templates", headers,
+        json={"mitre_technique_id": mitre_technique_id, "name": name},
+    )
+    assert resp.status_code == 201, resp.text
+    return resp.json()["id"]
+
+
+def test_count_matches_number_of_matching_templates(
+    client, db, api, auth_headers, red_lead_headers, technique,
+):
+    _create_template(api, red_lead_headers, "T1059.300", "Count test template A")
+    _create_template(api, red_lead_headers, "T1059.300", "Count test template B")
+
+    resp = api(
+        "get", "/api/v1/test-templates/count", auth_headers,
+        params={"mitre_technique_id": "T1059.300"},
+    )
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["total"] == 2
+
+
+def test_count_respects_search_filter(client, db, api, auth_headers, red_lead_headers, technique):
+    _create_template(api, red_lead_headers, "T1059.300", "Unique Zebra Template")
+    _create_template(api, red_lead_headers, "T1059.300", "Something else entirely")
+
+    resp = api("get", "/api/v1/test-templates/count", auth_headers, params={"search": "Zebra"})
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["total"] == 1
+
+
+def test_count_matches_list_length_when_under_page_size(
+    client, db, api, auth_headers, red_lead_headers, technique,
+):
+    _create_template(api, red_lead_headers, "T1059.300", "Parity check template")
+
+    listed = api(
+        "get", "/api/v1/test-templates", auth_headers,
+        params={"mitre_technique_id": "T1059.300"},
+    )
+    counted = api(
+        "get", "/api/v1/test-templates/count", auth_headers,
+        params={"mitre_technique_id": "T1059.300"},
+    )
+    assert len(listed.json()) == counted.json()["total"]
+
+
+def test_count_accessible_to_any_authenticated_role(client, db, api, red_tech_headers):
+    """Pagination is a basic UX need, not a lead-only privilege like /stats."""
+    resp = api("get", "/api/v1/test-templates/count", red_tech_headers)
+    assert resp.status_code == 200, resp.text