Vereinfacht client: email normalization, multi-match warning, Bypass tests, doc note

- Normalize email (trim + downcase) before filter lookup
- Log warning when API returns multiple contacts for same email
- Add Bypass tests for find_contact_by_email (query params, empty/single response parsing)
- Document vereinfacht_required_field? as legacy/unused in vereinfacht-api.md
- Add bypass dependency (dev+test) for HTTP stubbing

mix.exs

@@ -75,6 +75,7 @@ defmodule Mv.MixProject do
       {:bandit, "~> 1.5"},
       {:mix_audit, "~> 2.1", only: [:dev, :test], runtime: false},
       {:sobelow, "~> 0.14", only: [:dev, :test], runtime: false},
+      {:bypass, "~> 2.1", only: [:dev, :test]},
       {:credo, "~> 1.7", only: [:dev, :test], runtime: false},
       {:picosat_elixir, "~> 0.1"},
       {:ecto_commons, "~> 0.3"},