fix: declare ruff as a dev dependency

CI has no nix shell, and ruff was only ever a nix package, so both lint steps
died with "Failed to spawn: ruff". Pinned exactly: the formatter deciding what
is correct must be one version, or CI rejects what the dev shell produced.

backend/shell.nix

@@ -4,7 +4,8 @@ pkgs.mkShell {
   buildInputs = with pkgs; [
     # Python development environment for Chitai
     python313Packages.greenlet
-    python313Packages.ruff
+    # ruff is a dev dependency in pyproject.toml, not a shell package: CI has no nix, so a
+    # nix-only formatter is one CI cannot run, and two copies could disagree on formatting.
     uv
 
     # postgres database