Higher Koszul duality and $n$-affineness

We study $\mathbb{E}_n$-Koszul duality for pairs of algebras of the form $\mathrm{C}_{\bullet}(\Omega^{n}_*X;\Bbbk) \leftrightarrow \mathrm{C}^{\bullet}(X;\Bbbk)$, and the closely related question of $n$-affineness for Betti stacks. It was expected, but not known, that $\mathbb{E}_n$-Koszul duality should induce a kind of Morita equivalence between categories of iterated modules. We establish this rigorously by proving that the $(\infty,n)$-category of iterated modules over $\mathrm{C}_{\bullet}(\Omega_*^{n+1}X;\Bbbk)$ is equivalent to the $(\infty,n)$-category of quasi-coherent sheaves of $(\infty,n-1)$-categories on $\mathrm{cSpec}(\mathrm{C}^{\bullet}(X;\Bbbk))$, where $\mathrm{cSpec}(\mathrm{C}^{\bullet}(X;\Bbbk))$ is the cospectrum of $\mathrm{C}^{\bullet}(X;\Bbbk)$. By the monodromy equivalence, these categories are also equivalent to the category of higher local systems on $X$, $n\mathbf{LocSysCat}^{n-1}(X;\Bbbk)$. Our result is new already in the classical case $n=1$, although it can be seen to recover well known formulations of $\mathbb{E}_1$-Koszul duality as a Morita equivalence of module categories (up to appropriate completions of the $t$-structures). We also investigate (higher) affineness properties of Betti stacks. We give a complete characterization of $n$-affine Betti stacks, in terms of the $0$-affineness of their iterated loop space. As a consequence, we prove that $n$-truncated Betti stacks are $n$-affine; and that $\pi_{n+1}(X)$ is an obstruction to $n$-affineness.