The Nix build uses Elixir 1.20.2 whose type checker reports :oidcc as
"not loaded" for type inference. This causes:
1. Warnings for undefined Oidcc.Plug.* functions (promoted to errors by
   warnings_as_errors: true)
2. A hard type-checker error for the struct pattern %Token{id: %Token.Id{...}}

Fix by:
- Adding no_warn_undefined in mix.exs for oidcc/oidcc_plug Erlang apps and
  the specific Elixir modules that are flagged as undefined
- Replacing the struct pattern match with a generic map pattern
  %{id: %{claims: id_claims}} which is equivalent at runtime and avoids
  the type checker's struct verification"

The mixFodDeps hash in flake.nix was stale (from before the oidcc migration),
causing the Nix build to fetch old mix dependencies (still using plugoid).
Update to the correct hash that matches the new mix.lock with oidcc/oidcc_plug.

Also revert the incorrect no_warn_undefined workaround in mix.exs and the
weakened struct pattern in auth_controller.ex added in the previous session.