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.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

MicCheck.slnx

@@ -1,4 +1,11 @@
 <Solution>
+  <Folder Name="/Solution Items/">
+    <File Path=".editorconfig" />
+    <File Path=".gitignore" />
+    <File Path="CLAUDE.md" />
+    <File Path="docker-compose.yml" />
+    <File Path="readme.md" />
+  </Folder>
   <Folder Name="/src/">
     <Project Path="src/admin/Admin.esproj">
       <Build />
@@ -10,5 +17,6 @@
   </Folder>
   <Folder Name="/tests/">
     <Project Path="tests/api/MicCheck.Api.Tests.Unit/MicCheck.Api.Tests.Unit.csproj" />
+    <Project Path="tests/api/MicCheck.Api.Tests.Integration/MicCheck.Api.Tests.Integration.csproj" />
   </Folder>
 </Solution>