fix: bodyless posts no longer send json content-type (fastify 400 on empty body)

web/test/api.test.ts

@@ -40,4 +40,19 @@ describe('api additions', () => {
     expect(err).toBeInstanceOf(ApiError)
     expect((err as ApiError).code).toBe('no_subsonic_config')
   })
+
+  it('bodyless posts do not send a json content-type (Fastify 400s on empty json bodies)', async () => {
+    fetchMock.mockClear()
+    fetchMock.mockImplementation(() => jsonOk({ ok: true }))
+    await api.returnLoan(7)
+    await api.markPlayed('a1')
+    await api.logout()
+    await api.triggerBackup()
+    for (const call of fetchMock.mock.calls) {
+      const init = call[1] as RequestInit | undefined
+      const headers = (init?.headers ?? {}) as Record<string, string>
+      expect(headers['Content-Type']).toBeUndefined()
+      expect(init?.body).toBeUndefined()
+    }
+  })
 })