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_t111_test_templates_router.py

@@ -182,11 +182,14 @@ def test_soft_delete():
 
 
 def test_search_filter():
-    from app.routers.test_templates import list_templates
-    source = inspect.getsource(list_templates)
+    from app.services.test_template_service import _build_template_query, list_templates
+    # The actual filter-building (including search) now lives in the shared
+    # _build_template_query() helper, reused by both list and count.
+    source = inspect.getsource(_build_template_query)
     assert "search" in source
     assert "name" in source and "description" in source
     assert "ilike" in source
+    assert "_build_template_query" in inspect.getsource(list_templates)
     print("  [PASS] Search filter searches in name and description")