fix(strommix): pick price at "now" + surface cross-border timestamp

Audit against ground truth at 22:15 lokal exposed three real bugs:

1. Day-ahead price was wrong (widget 125, real 153 €/MWh). The CMS'
   `last_non_null` transform always picks the last cell of the 96-slot
   array — typically a slot in the future, not the slot covering "now".
   Bypass the CMS for prices: aggregator fetches energy-charts /price
   directly, walks unix_seconds, and picks the largest start ≤ now.
   `priceMeasuredAtUnix` is surfaced so the widget can show the slot
   timestamp as a sanity check.

2. Cross-border saldo (cbpf) typically lags 1.5–2 h. Widget labelled
   it as "jetzt" even when the upstream's last datapoint was 90 min
   old. Now the cross-border timestamp travels through the aggregator
   and the widget renders "Stand 20:30 Uhr · verzögert" when the gap
   to the production snapshot exceeds 30 min.

3. Bucket label "Dunkelflaute" misuses the term — the formal
   definition needs ≥24 h history, not a single threshold crossing.
   Updated the strommix-default content entry to use "Sehr niedrige
   Einspeisung" / "Very low feed-in" instead, and explained the
   reasoning in a comment so editors don't re-introduce the misnomer.

Aggregator response shape gained `crossborderMeasuredAtUnix` +
`priceMeasuredAtUnix`; the widget renders both as small timestamps
under their values, plus a "verzögert" hint when cross-border data is
older than the 30-min tolerance window.

Cache TTLs on the price external collections lowered from 30 min to
15 min so the admin-side cache flips at the same cadence as the
day-ahead slots (since 30.09.2025 prices come in 15-min buckets).

src/routes/api/strommix/+server.ts

@@ -53,11 +53,6 @@ type CrossborderRaw = {
   measured_at_unix?: number | null;
 };
 
-type PriceRaw = {
-  price_eur_mwh?: number | null;
-  measured_at_unix?: number | null;
-};
-
 function num(x: unknown): number {
   return typeof x === "number" && Number.isFinite(x) ? x : 0;
 }
@@ -80,12 +75,62 @@ async function fetchOne<T>(
   }
 }
 
+/**
+ * Day-ahead prices come as a 96-slot array (15-min intervals over 24 h)
+ * and the CMS' `last_non_null` transform always picks the LAST known
+ * slot — typically a slot in the future, not the one covering "now".
+ * Bypass the CMS for prices and pick the slot whose start time is the
+ * largest one ≤ now. Returns null when the upstream lacks a usable
+ * value.
+ */
+type PriceArrayResp = {
+  unix_seconds?: number[];
+  price?: (number | null)[];
+};
+
+async function fetchPriceAtNow(
+  bzn: string,
+  fetcher: typeof fetch
+): Promise<{ price: number | null; slotUnix: number | null; stale: boolean }> {
+  try {
+    const res = await fetcher(
+      `https://api.energy-charts.info/price?bzn=${encodeURIComponent(bzn)}`,
+      { signal: AbortSignal.timeout(8_000) }
+    );
+    if (!res.ok) return { price: null, slotUnix: null, stale: false };
+    const data = (await res.json()) as PriceArrayResp;
+    const times = data.unix_seconds ?? [];
+    const prices = data.price ?? [];
+    if (times.length === 0 || prices.length === 0) {
+      return { price: null, slotUnix: null, stale: false };
+    }
+    const nowSec = Math.floor(Date.now() / 1000);
+    let pickIdx = -1;
+    for (let i = 0; i < times.length; i++) {
+      if (times[i] <= nowSec) pickIdx = i;
+      else break;
+    }
+    if (pickIdx < 0) return { price: null, slotUnix: null, stale: false };
+    const value = prices[pickIdx];
+    return {
+      price: typeof value === "number" && Number.isFinite(value) ? value : null,
+      slotUnix: times[pickIdx] ?? null,
+      stale: false,
+    };
+  } catch {
+    return { price: null, slotUnix: null, stale: true };
+  }
+}
+
 export const GET: RequestHandler = async ({ fetch }) => {
   const [strommix, cross, priceDe, priceFr] = await Promise.all([
     fetchOne<StrommixRaw>("strommix_de", fetch),
     fetchOne<CrossborderRaw>("crossborder_de", fetch),
-    fetchOne<PriceRaw>("price_de", fetch),
-    fetchOne<PriceRaw>("price_fr", fetch),
+    // Prices fetched directly so we can index by "now" instead of
+    // taking the last (potentially future) slot the CMS' last_non_null
+    // transform would pick.
+    fetchPriceAtNow("DE-LU", fetch),
+    fetchPriceAtNow("FR", fetch),
   ]);
 
   const s = strommix.data ?? {};
@@ -125,9 +170,11 @@ export const GET: RequestHandler = async ({ fetch }) => {
     weatherRenewableMw,
     conventionalMw,
     netFlowGw,
+    crossborderMeasuredAtUnix: c.measured_at_unix ?? null,
     flowsByCountry,
-    priceDeEurMwh: priceDe.data?.price_eur_mwh ?? null,
-    priceFrEurMwh: priceFr.data?.price_eur_mwh ?? null,
+    priceDeEurMwh: priceDe.price,
+    priceFrEurMwh: priceFr.price,
+    priceMeasuredAtUnix: priceDe.slotUnix,
     staleStrommix: strommix.stale,
     staleCrossborder: cross.stale,
     stalePrice: priceDe.stale || priceFr.stale,