paperless: fix cors header

`django-cors-headers` 3.x (which is used in nixpkgs) requires a scheme
for allowed hosts. Upstream uses 2.4, however we create the python env
with Nix, so the source needs to be patched accordingly.

pkgs/applications/office/paperless/default.nix

@@ -57,6 +57,12 @@ let
       cp -r --no-preserve=mode $src/src/* $src/LICENSE $srcDir
     '';
 
+    postPatch = ''
+      # django-cors-headers 3.x requires a scheme for allowed hosts
+      substituteInPlace $out/share/paperless/paperless/settings.py \
+        --replace "localhost:8080" "http://localhost:8080"
+    '';
+
     buildPhase = let
       # Paperless has explicit runtime checks that expect these binaries to be in PATH
       extraBin = lib.makeBinPath [ imagemagick7 ghostscript optipng tesseract unpaper ];