Set up nix-shell + fastapi base

shell.nix

@@ -0,0 +1,17 @@
+# run with "nix develop -f shell.nix"
+let
+  # We pin to a specific nixpkgs commit for reproducibility.
+  # Last updated: 2026-04-12. Check for new commits at https://status.nixos.org.
+  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/54170c54449ea4d6725efd30d719c5e505f1c10e.tar.gz") {};
+in pkgs.mkShell {
+  packages = [
+    (pkgs.python3.withPackages (python-pkgs: with python-pkgs; [
+      # select Python packages here
+      fastapi
+      pydantic
+      sqlalchemy
+      uvicorn
+    ]))
+    pkgs.fastapi-cli
+  ];
+}