Add Web Push warning notification backend

lib/server-warnings.ts

@@ -0,0 +1,15 @@
+import { normalizeWarning } from "@/lib/weather-utils";
+import type { RawWarning, WeatherWarning } from "@/types/imgw";
+
+const IMGW_WARNINGS_METEO_URL = "https://danepubliczne.imgw.pl/api/data/warningsmeteo";
+
+export async function fetchMeteoWarnings(signal?: AbortSignal): Promise<WeatherWarning[]> {
+  const response = await fetch(IMGW_WARNINGS_METEO_URL, {
+    headers: { accept: "application/json" },
+    next: { revalidate: 300 },
+    signal,
+  });
+  if (!response.ok) throw new Error("Unable to load IMGW meteorological warnings.");
+  const rows = await response.json() as RawWarning[];
+  return Array.isArray(rows) ? rows.map((warning, index) => normalizeWarning(warning, "meteo", index)) : [];
+}