fix(registry): clean content_repo failure in hygiene; retire META_REPO from .env.example; clarity + idempotency test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_multi_project_spine_vertical.py

@@ -110,15 +110,21 @@ def test_registry_mirror_is_idempotent(app_with_fake_gitea):
     """Re-running the registry mirror is safe — it upserts (overwrites) the
     projects row from projects.yaml each time without raising. M3 retirement
     of seed_default_project: the registry mirror is now the sole authority."""
-    from app import db
+    import asyncio
+    from app import db, registry as registry_mod
 
     app, _ = app_with_fake_gitea
     with TestClient(app):
-        # After startup the row should have content_repo from projects.yaml.
-        got = db.conn().execute(
-            "SELECT content_repo FROM projects WHERE id = 'default'"
-        ).fetchone()["content_repo"]
-        assert got == "meta"
+        before = db.conn().execute("SELECT COUNT(*) AS n FROM projects").fetchone()["n"]
+        cfg = app.state.config
+        gitea = app.state.gitea
+        asyncio.run(registry_mod.refresh_registry(cfg, gitea))
+        after = db.conn().execute("SELECT COUNT(*) AS n FROM projects").fetchone()["n"]
+        assert after == before
+        row = db.conn().execute(
+            "SELECT content_repo FROM projects WHERE id='default'"
+        ).fetchone()
+        assert row["content_repo"] == "meta"
 
 
 def test_registry_repo_config_wired(app_with_fake_gitea, monkeypatch):