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>

backend/src/app.ts

@@ -86,10 +86,11 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
   const authenticate = authPreHandler(config.authMode);
   app.addHook("preHandler", async (req, reply) => {
     // Process readiness is intentionally unauthenticated for local container/proxy probes.
-    if (req.url === "/health") return;
+    if (req.url === "/health" || req.url === "/health/dwh") return;
     return authenticate(req, reply);
   });
   app.get("/health", async () => ({ status: "ok" }));
+  app.get("/health/dwh", async () => tht.dbPing());
   app.get("/me", async (req) => getPrincipal(req));
   sessionRoutes(app, {
     mgr, tht: tht as ThtRunner, hub, getSettings, readiness, dwhPrecheck: config.dwhPrecheck,