feat: materials by zjazd in the student API and admin

The material admin form gets the "Na zjazd" picker, every student
material JSON carries meeting_id, and GET /materials accepts meeting=
inside the published-only scope, so the Pulpit hero can list the
zjazd's materials next to its readings.

controllers/api/handlers_test.go

@@ -148,6 +148,7 @@ func TestQueryValidation(t *testing.T) {
 		{"materials page zero", h.Materials, "/?page=0", "page"},
 		{"materials per_page too large", h.Materials, "/?per_page=1000", "per_page"},
 		{"materials course negative", h.Materials, "/?course=-1", "course"},
+		{"materials meeting zero", h.Materials, "/?meeting=0", "meeting"},
 		{"materials tag too long", h.Materials, "/?tag=" + longTag(), "tag"},
 	}
 	for _, tc := range cases {
@@ -188,6 +189,7 @@ func TestValidQueriesReachTheDatabase(t *testing.T) {
 		{h.Readings, "/?course=3"},
 		{h.Courses, "/?semester=2"},
 		{h.Materials, "/?course=1&tag=geopolityka&type=pdf&page=2&per_page=10"},
+		{h.Materials, "/?meeting=4&per_page=100"},
 	} {
 		if code, body := serve(t, tc.fn, http.MethodGet, tc.target, user, nil); code != http.StatusServiceUnavailable {
 			t.Errorf("%s = %d %v, want to pass validation (503 without a database)", tc.target, code, body)