Point email links at conchecker.scoutitsystems.com and show build SHA in header.

APP_BASE_URL default (config, .env.example, both compose files) is now
https://conchecker.scoutitsystems.com with no port, so review-required
and final-report email links use the public site. CI bakes the short
commit SHA into the image as APP_BUILD via a Docker build-arg; /health
returns version+build and the site header shows the build so it's easy
to confirm which image is deployed. Local runs default to 'dev'.

tests/api/test_build_info.py

@@ -0,0 +1,21 @@
+from fastapi.testclient import TestClient
+
+from backend import config
+from backend.main import app
+
+
+def test_health_includes_version_and_build():
+    client = TestClient(app)
+    response = client.get("/health")
+    assert response.status_code == 200
+    body = response.json()
+    assert body["version"] == config.APP_VERSION
+    assert body["build"] == config.APP_BUILD
+
+
+def test_app_base_url_default_is_public_site():
+    assert config.APP_BASE_URL == "https://conchecker.scoutitsystems.com"
+
+
+def test_app_build_defaults_to_dev():
+    assert config.APP_BUILD == "dev"