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>

frontend/src/api/client.ts

@@ -1,5 +1,17 @@
 import { backendBaseUrl as BASE, joinBackendPath } from "./runtime-config";
 
+/**
+ * Error thrown for non-2xx responses. `.message` stays "<status> <body>" for
+ * backward compatibility; `.status` and `.payload` (parsed JSON body, if any)
+ * let callers branch on a specific failure — e.g. a `code: "dwh_unreachable"`.
+ */
+export class ApiError extends Error {
+  constructor(readonly status: number, readonly bodyText: string, readonly payload: unknown) {
+    super(`${status} ${bodyText}`);
+    this.name = "ApiError";
+  }
+}
+
 export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
   // Only declare a JSON content-type when we actually send a body. Body-less
   // POSTs (resume, close) would otherwise make Fastify reject the empty body
@@ -11,7 +23,12 @@ export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T>
     headers["content-type"] = "application/json";
   }
   const res = await fetch(joinBackendPath(BASE, path), { ...init, headers });
-  if (!res.ok) throw new Error(`${res.status} ${await res.text().catch(() => "")}`);
+  if (!res.ok) {
+    const bodyText = await res.text().catch(() => "");
+    let payload: unknown;
+    try { payload = bodyText ? JSON.parse(bodyText) : undefined; } catch { payload = undefined; }
+    throw new ApiError(res.status, bodyText, payload);
+  }
   if (res.status === 204) return undefined as T;
   // Accepted fire-and-forget endpoints may legitimately return 202 with no
   // representation. Keep apiFetch useful for both 202 and 204 contracts.