feat: pre-check DWH reachability before creating a session (local dev only)

New session now refuses to spawn a Pi runtime that would only die in bootstrap
retrieval when the DWH/vector host is unreachable (e.g. a dropped VPN). Before
`session new`, POST /sessions probes the DWH via `tht db ping`; if it is down it
returns 503 {code:"dwh_unreachable"} with a clear message and creates nothing.

- Gated behind the THT_DWH_PRECHECK flag (default off), enabled only by the local
  dev launcher (run-stack.sh) — containers/CI never pay the probe, and existing
  tests that don't set it are unaffected.
- ThtRunner.dbPing() runs `tht db ping` with a 10s timeout (run() gains an optional
  timeout that SIGKILLs a hung child).
- Frontend: apiFetch throws a typed ApiError (status + parsed payload); the new-
  session composer shows the specific alert on `dwh_unreachable` instead of the
  generic retry hint, keeping the question for retry.

Verified live on an isolated backend (precheck on + broken DWH host → 503
dwh_unreachable, no session created) and via unit tests (backend 228, frontend 308).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/test/routes-sessions.test.ts

@@ -178,6 +178,63 @@ test("POST /sessions usa i settings (workspace/provider/model/thinking) e crea+a
   unlinkSync(modelKey);
 });
 
+test("POST /sessions refuses to create a session when the local DWH precheck fails", async () => {
+  let sessionNewCalls = 0;
+  let pinged = 0;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness", THT_DWH_PRECHECK: "1" }), {
+    thtRunner: {
+      dbPing: async () => { pinged += 1; return { ok: false, detail: "DWH non accessibile" }; },
+      sessionNew: async () => { sessionNewCalls += 1; return { id: "s1" }; },
+      searchPack: async () => {},
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "w" }) as any,
+  });
+  const res = await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+  expect(res.statusCode).toBe(503);
+  expect(res.json()).toMatchObject({ code: "dwh_unreachable" });
+  expect(pinged).toBe(1);
+  expect(sessionNewCalls).toBe(0); // session must not be created
+});
+
+test("POST /sessions proceeds past a passing DWH precheck", async () => {
+  let pinged = 0;
+  let created = 0;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness", THT_DWH_PRECHECK: "1" }), {
+    thtRunner: {
+      dbPing: async () => { pinged += 1; return { ok: true, detail: "OK" }; },
+      sessionNew: async () => { created += 1; return { id: "s1" }; },
+      searchPack: async () => {},
+      sessionList: async () => [{ id: "s1" }],
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "w" }) as any,
+    spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
+  });
+  const res = await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+  expect(res.statusCode).toBe(200);
+  expect(pinged).toBe(1);
+  expect(created).toBe(1);
+});
+
+test("POST /sessions skips the DWH precheck when the flag is off (default)", async () => {
+  let pinged = 0;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    thtRunner: {
+      dbPing: async () => { pinged += 1; return { ok: false, detail: "should never run" }; },
+      sessionNew: async () => ({ id: "s1" }),
+      searchPack: async () => {},
+      sessionList: async () => [{ id: "s1" }],
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "w" }) as any,
+    spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
+  });
+  const res = await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+  expect(res.statusCode).toBe(200);
+  expect(pinged).toBe(0); // probe never runs without the flag
+});
+
 test("POST /sessions configura Pi con il thinking globale selezionato", async () => {
   let configured: any;
   const bridge = { onClientEvent: () => {}, emitClientEvent: () => {} };