feat(backend): Ollama embeddings preflight on session create/resume (503 hard-fail)

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

backend/test/e2e-f1.test.ts

@@ -10,6 +10,7 @@ const SCRIPT = path.resolve("../harness/tests/fake_pi/scripts/f1_disambiguation.
 test("loop F1: crea sessione → SSE riceve il widget → risponde → 204", async () => {
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     thtRunner: {
+      ollamaEnsure: async () => ({ ok: true }),
       sessionNew: async () => ({ id: "s1" }),
       sessionShow: async (_id: string) => ({ id: "s1", provider: undefined, model: undefined, thinking: undefined }),
       sessionList: async () => [],