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

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

backend/src/routes/sessions.ts

@@ -7,11 +7,13 @@ import { getUser } from "../auth/auth.js";
 
 export function sessionRoutes(
   app: FastifyInstance,
-  d: { mgr: PiProcessManager; tht: ThtRunner; hub: SseHub; getSettings: () => Settings },
+  d: { mgr: PiProcessManager; tht: ThtRunner; hub: SseHub; getSettings: () => Settings; ollamaEnsureTimeoutSec: number },
 ) {
   app.post("/sessions", async (req, reply) => {
     const b = req.body as { question: string; name?: string };
     const s = d.getSettings();
+    const ensure = await d.tht.ollamaEnsure(s.workspace ?? "", d.ollamaEnsureTimeoutSec);
+    if (!ensure.ok) return reply.code(503).send({ error: ensure.error ?? "Ollama/embeddings non disponibili" });
     // Settings (global) supply workspace/provider/model/thinking. The new-question
     // form sends only the question text. `workspace` selects the tht `-c <config>`.
     const { id } = await d.tht.sessionNew({
@@ -48,6 +50,8 @@ export function sessionRoutes(
   });
   app.post("/sessions/:id/resume", async (req, reply) => {
     const id = (req.params as any).id;
+    const ensure = await d.tht.ollamaEnsure(d.getSettings().workspace ?? "", d.ollamaEnsureTimeoutSec);
+    if (!ensure.ok) return reply.code(503).send({ error: ensure.error ?? "Ollama/embeddings non disponibili" });
     const manifest = (await d.tht.sessionShow(id)) as { status?: string; archived?: boolean } | null;
     if (manifest?.status === "finalized" || manifest?.archived) {
       return reply.code(409).send({ error: "sessione in sola lettura (finalizzata o archiviata)" });