feat: DWH connectivity probe at startup — modal alert within 5s if unreachable

Backend: GET /health/dwh (unauthenticated) calls tht db ping with a 5s
timeout. Frontend: checkDwhHealth() races a 5s timer against the fetch;
on failure a non-dismissable Dialog with Retry appears immediately.

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

frontend/src/api/sessions.ts

@@ -53,3 +53,15 @@ export const deleteSession = (id: string) =>
 
 export const getSessionDocuments = (id: string) =>
   apiFetch<SessionDocument[]>(`/sessions/${id}/documents`);
+
+export async function checkDwhHealth(): Promise<{ ok: boolean }> {
+  const timeout = new Promise<{ ok: false }>((r) => setTimeout(() => r({ ok: false }), 5000));
+  try {
+    return await Promise.race([
+      apiFetch<{ ok: boolean; detail: string }>("/health/dwh"),
+      timeout,
+    ]);
+  } catch {
+    return { ok: false };
+  }
+}