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/components/blocks/StrommixBlock.svelte

@@ -99,6 +99,15 @@
     return d.toLocaleTimeString("de-DE", { hour: "2-digit", minute: "2-digit" });
   }
 
+  /** Cross-border data is considered stale (and worth flagging) when its
+   *  timestamp is more than 30 min behind the production-snapshot. cbpf
+   *  typically lags 1.5–2 h, so this is the rule rather than the
+   *  exception — surfacing the stamp keeps the widget honest. */
+  const cbStale = $derived.by(() => {
+    if (!live?.measuredAtUnix || !live?.crossborderMeasuredAtUnix) return false;
+    return live.measuredAtUnix - live.crossborderMeasuredAtUnix > 30 * 60;
+  });
+
   const bucketAccent = $derived.by(() => {
     if (!bucket) return "neutral";
     return bucket === "dunkelflaute"
@@ -198,6 +207,7 @@
         <div class="px-4 py-3">
           <div class="text-xs text-neutral-600 uppercase tracking-wide">Konventionell jetzt</div>
           <div class="text-lg font-semibold tabular-nums">{fmtMw(live.conventionalMw)}</div>
+          <div class="text-[11px] text-neutral-500 mt-0.5">Braunkohle + Steinkohle + Erdgas + Kernkraft</div>
         </div>
         <div class="px-4 py-3">
           <div class="text-xs text-neutral-600 uppercase tracking-wide">
@@ -208,6 +218,12 @@
             {#if importDirection === "Export"}↑{/if}
             {fmtGw(live.netFlowGw)}
           </div>
+          {#if live.crossborderMeasuredAtUnix}
+            <div class="text-[11px] text-neutral-500 mt-0.5">
+              Stand: {fmtTime(live.crossborderMeasuredAtUnix)} Uhr
+              {#if cbStale}<span class="text-amber-700"> · verzögert</span>{/if}
+            </div>
+          {/if}
         </div>
       </div>
 
@@ -218,6 +234,11 @@
           {#if enableFr && live.priceFrEurMwh != null}
             <span class="text-neutral-600">(Frankreich: <span class="tabular-nums">{fmtPrice(live.priceFrEurMwh)}</span>)</span>
           {/if}
+          {#if live.priceMeasuredAtUnix}
+            <span class="text-[11px] text-neutral-500">
+              · Slot {fmtTime(live.priceMeasuredAtUnix)}
+            </span>
+          {/if}
           {#if (live.priceDeEurMwh ?? 0) < 0}
             <span class="ml-auto inline-flex items-center gap-1 text-amber-800 font-semibold">
               <Icon icon="mdi:alert-circle-outline" class="size-4" aria-hidden="true" />