Add MicCheck.Api.Tests.Integration project

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.

tests/api/MicCheck.Api.Tests.Integration/HttpFiles/Flags.http

@@ -0,0 +1,4 @@
+### GetFlags
+GET {{baseUrl}}/api/v1/flags
+X-Environment-Key: {{environmentKey}}
+Accept: application/json