test(i18n): a 404 target the scan cannot read fails the backend-id contract

what=SOME_CONSTANT reached no locale check at all; only _common.py's own
pass-through may name the target by variable.

frontend/src/i18n/backendIds.contract.test.ts

@@ -88,16 +88,26 @@ const DETAIL = "downloads.errors.fail.catalog.";
 
 // What a structured 404 says is gone → `errors.notFound.<what>`: every `not_found("x")` and every
 // `owned_or_404(..., what="x")` / `get_or_404(..., what="x")` under routes/, read from the sources.
-const routeSources = (dir = "routes"): string[] =>
+const routeFiles = (dir = "routes"): string[] =>
   readdirSync(new URL(`../../../backend/app/${dir}/`, import.meta.url), {
     withFileTypes: true,
   }).flatMap((e) =>
     e.isDirectory()
-      ? routeSources(`${dir}/${e.name}`)
+      ? routeFiles(`${dir}/${e.name}`)
       : e.name.endsWith(".py")
-        ? [backend(`${dir}/${e.name}`)]
+        ? [`${dir}/${e.name}`]
         : [],
   );
+const routeSources = (): string[] => routeFiles().map(backend);
+// A target the scan cannot read — `what=SOME_CONSTANT` — is a 404 no locale is checked for (one hid
+// `debug_session`). Only the helpers' own pass-through in `_common.py` may name it by variable.
+const opaqueWhats = routeFiles()
+  .filter((f) => !f.endsWith("/_common.py"))
+  .flatMap((f) =>
+    [...backend(f).matchAll(/not_found\((?!"|\))[^)]*\)|what=(?!")\S+/g)].map(
+      (m) => `${f}: ${m[0]}`,
+    ),
+  );
 const goneWhats = [
   ...new Set(
     routeSources().flatMap((src) =>
@@ -158,6 +168,7 @@ describe("backend ids have their names in both languages", () => {
     expect(adminCodes).toEqual(expect.arrayContaining(["activate", "setpw"]));
     expect(failDetails).toEqual(expect.arrayContaining(["facebook.login_wall"]));
     expect(goneWhats).toEqual(expect.arrayContaining(["playlist", "download", "job"]));
+    expect(opaqueWhats).toEqual([]);
     expect(youtubeReasons).toEqual(["quota", "reconnect", "unavailable"]);
     expect(refusedReasons).toEqual(
       expect.arrayContaining(["weak_password", "setting_too_low", "plex_write_failed"]),