feat(slice-4): VERSION file + versioned /healthz — the flotilla verify contract (§8.1)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/tests/test_bootstrap.py

@@ -12,7 +12,7 @@ def test_inv_1_bootstrap_whole_flow_from_empty(fresh_db_url):
     # fresh_db_url is a brand-new empty database; create_app() self-migrates (INV-7).
     with TestClient(create_app(database_url=fresh_db_url)) as client:
         # a fresh deployment serves healthz green before any row exists
-        assert client.get("/healthz").json() == {"status": "ok"}
+        assert client.get("/healthz").json()["status"] == "ok"
 
         # PUC-2: first visitor requests a code; it reaches them via the dev channel
         assert client.post(