Tests: route Yavsc.Org test host through Testing environment

TestWebApplicationFactory used ASPNETCORE_ENVIRONMENT=Development, which
caused Program.Main's AddConfiguration("org") to load the tracked
appsettings-org.json (the reference file with the
'*** via dotnet user-secrets ou variable d'environnement ***'
placeholder connection string). Npgsql then failed to parse that
placeholder during host startup, failing six integration tests
(observed 2026-07-11: System.ArgumentException on
NpgsqlConnectionStringBuilder.set_Item).

Switching the test host to a dedicated Testing environment makes
AddConfiguration("org") pick up the new optional
appsettings-org.Testing.json file as the last source in the chain
(JSON → env vars), which overrides YavscConnection with the
InMemory marker and the Smtp section with the test stub values.

The .gitignore exception whitelists this file explicitly: it is a
configuration source for the test host, not a secrets file.

The WebServerFixture path is unchanged — it owns its
WebApplicationBuilder and adds the same in-memory override via its
BuildApp hook.

.gitignore

@@ -24,6 +24,15 @@ data/
 appsettings.*.json
 appsettings-*.*.json
 
+# Exception: the Testing-environment override for Yavsc.Org is a tracked
+# configuration source, not a secrets file. TestWebApplicationFactory
+# (Yavsc.Org.Tests) flips ASPNETCORE_ENVIRONMENT to "Testing" so
+# AddConfiguration("org") in Program.Main loads this file as the
+# last in the chain (it is optional). It overrides the connection
+# string and SMTP section for the in-memory test host and contains
+# no production secrets.
+!src/Yavsc.Org/appsettings-org.Testing.json
+
 generated/
 *.tmp
 DataDir/