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/lib/strommix.ts

@@ -7,6 +7,9 @@
  */
 
 export type StrommixResponse = {
+  // Production-snapshot timestamp (Unix sec) — used as the "anchor" for
+  // the rest of the values. Cross-border + prices may have their own,
+  // earlier or later, timestamps.
   measuredAtUnix: number | null;
   // Production (MW)
   windOnshoreMw: number;
@@ -24,10 +27,18 @@ export type StrommixResponse = {
   conventionalMw: number;
   // Cross-border (GW; positive = net export from DE, negative = import)
   netFlowGw: number;
+  /** When was the cross-border saldo last reported by upstream? cbpf
+   *  typically lags 1–2 h behind real time. Widget compares to
+   *  `measuredAtUnix` to decide whether to show "Stand HH:MM". */
+  crossborderMeasuredAtUnix: number | null;
   flowsByCountry: Record<string, number>;
-  // Prices (EUR/MWh; null if upstream unavailable)
+  // Prices (EUR/MWh; null if upstream unavailable). Picked from the
+  // 15-min day-ahead slot whose interval contains "now", not the last
+  // non-null cell of the array.
   priceDeEurMwh: number | null;
   priceFrEurMwh: number | null;
+  /** Unix sec of the price-slot start that produced `priceDeEurMwh`. */
+  priceMeasuredAtUnix: number | null;
   // Stale flags surfaced from the CMS' `x-rustycms-external-stale` header
   staleStrommix: boolean;
   staleCrossborder: boolean;