Remove duplicate source files from old ApiKeys/, Authentication/, and
Authorization/ folders that were already migrated to Common/Security/.
Update test namespace usings to match. Pin EFC Relational 10.0.5 to
resolve MSB3277 version conflict with Npgsql's transitive dependency.
Add LangVersion and TreatWarningsAsErrors to AppHost and ServiceDefaults.
NUnit integration tests that target a running API by URL and seed/clean
data directly in Postgres. Settings (BaseUrl, DbConnectionString) come
from .runsettings or environment variables so tests can be pointed at
any environment. HTTP requests live in HttpFiles/*.http; assertion
failures include a fresh DB snapshot of the relevant rows. First test
covers the flag-enabled path.
