🛂(y-provider) verify jwt token instead of the shared api key

The /api/convert route no longer accepts the Y_PROVIDER_API_KEY shared
secret. It now verifies the admin JWT signed by Django against the
JWKS published on its /api/v1.0/jwks endpoint.

src/frontend/servers/y-provider/__tests__/convert.test.ts

@@ -17,6 +17,8 @@ vi.mock('../src/env', async (importOriginal) => {
   };
 });
 
+import { mockJwksEndpoint, signAdminToken } from './testUtils/adminJwt';
+
 import { docsBlockNoteSchema } from '@/blockSpecs';
 import { initApp } from '@/servers';