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/src/routes/sessions.ts

@@ -13,6 +13,8 @@ const READINESS_FAILURE_MESSAGE =
   "Session services are not ready. Check configuration and connectivity, then try again.";
 const RESUME_FAILURE_MESSAGE =
   "Session could not be resumed. Check configuration and connectivity, then try again.";
+const DWH_UNREACHABLE_MESSAGE =
+  "Cannot start a session: the data warehouse is unreachable. Check the VPN connection and try again.";
 
 function eventCursor(...values: unknown[]): number {
   let cursor = 0;
@@ -30,6 +32,8 @@ export function sessionRoutes(
     mgr: PiProcessManager; tht: ThtRunner; hub: SseHub;
     getSettings: (principal: PrincipalContext) => Promise<Settings>;
     readiness: ReadinessManager;
+    /** Local-only guard: probe DWH reachability before creating a session (run-stack.sh). */
+    dwhPrecheck?: boolean;
   },
 ) {
   const lifecycleTails = new Map<string, Promise<void>>();
@@ -181,6 +185,16 @@ export function sessionRoutes(
     const runner = runnerFor(principal);
     const ensure = await d.readiness.ensure(s.workspace ?? "", principal);
     if (!ensure.ok) return reply.code(503).send({ error: READINESS_FAILURE_MESSAGE });
+    // Local-only: verify the DWH is reachable BEFORE creating the session, so a dropped
+    // VPN surfaces as an up-front alert instead of a session that spawns Pi and then dies
+    // in bootstrap retrieval. `code` lets the client show a specific message.
+    if (d.dwhPrecheck) {
+      const ping = await runner.dbPing(s.workspace);
+      if (!ping.ok) {
+        console.error(`[dwh-precheck] refusing new session — DWH unreachable: ${ping.detail}`);
+        return reply.code(503).send({ error: DWH_UNREACHABLE_MESSAGE, code: "dwh_unreachable" });
+      }
+    }
     // Settings (global) supply workspace/provider/model/thinking. The new-question
     // form sends only the question text. `workspace` selects the tht `-c <config>`.
     let id: string;