fix(nix): correct cargoBuildFlags package name zesdex-backend -> zesdex-gateway

The gateway crate's package name is zesdex-gateway (not zesdex-backend);
the stale -p flag caused  to fail in CI with
'package ID specification zesdex-backend did not match any packages'.
This unblocks the GHA-only deploy workflow.

flake.nix

@@ -42,7 +42,7 @@
           };
 
           # Build only the main binary
-          cargoBuildFlags = [ "-p" "zesdex-backend" "--bin" "zesdex" ];
+          cargoBuildFlags = [ "-p" "zesdex-gateway" "--bin" "zesdex" ];
 
           nativeBuildInputs = with pkgs; [ pkg-config ];
           buildInputs = with pkgs; [ openssl sqlite ];