feat(strommix): history-driven panels (residual load, dunkelflaute, neg-price, flow balance)

Aggregator route /api/strommix now reads three RustyCMS history-mode
collections in parallel — strommix_history_de, price_de_history,
crossborder_history_de — and projects derived metrics on top of the raw
upstream snapshots stored there:

- todayHourly: per-slot Last + Wind+Solar for the residual-load chart
- negPriceHoursYtd: count of YTD slots where DE-LU day-ahead < 0
- crossborderTodayGwh / netCrossborderGwhYtd / crossborderByCountryGwhYtd:
  GW × slot-hours integrated into a per-country GWh balance
- lastDunkelflaute / worstDunkelflauten: contiguous runs where
  (wind+solar)/load drops below 10 % for at least 1 h, top 5 by duration

All four are null-graceful — collections that the CMS hasn't backfilled
yet leave the corresponding panel hidden instead of breaking the widget.

Five new components in lib/components/blocks/strommix/:
- ResidualLoadChart.svelte — SVG line chart, no chart-lib dep
- CapacityFactorBars.svelte — live vs installed Wind/Wind-Off/Solar
- DunkelflauteCounter.svelte — last phase card + top-5 list
- NegPriceCounter.svelte — YTD hours + manual EinsMan compensation
- FlowBalanceTodayYtd.svelte — today + YTD saldo with per-country breakdown

Block schema additions (block-types.ts + strommix-default.json5 in
cms_content): installed_*_gw / installed_as_of for the capacity-factor
bars (BNetzA Marktstammdatenregister snapshot, manually maintained), and
einsman_costs_ytd_eur_mio / einsman_costs_as_of for the NegPrice panel
(BNetzA quarterly report figures, no API).

src/lib/components/blocks/strommix/NegPriceCounter.svelte

@@ -0,0 +1,75 @@
+<script lang="ts">
+    /** Stunden mit negativem Day-Ahead-Preis YTD + manuell gepflegte
+     *  EinsMan-Entschädigungssumme aus dem Block. */
+    type Props = {
+        negHoursYtd: number | null;
+        einsmanEurMio: number | null;
+        einsmanAsOf: string | undefined;
+    };
+
+    let { negHoursYtd, einsmanEurMio, einsmanAsOf }: Props = $props();
+
+    function fmtNum(n: number, fractionDigits = 0): string {
+        return n.toLocaleString("de-DE", {
+            minimumFractionDigits: fractionDigits,
+            maximumFractionDigits: fractionDigits,
+        });
+    }
+
+    const showNeg = $derived(typeof negHoursYtd === "number");
+    const showEinsman = $derived(typeof einsmanEurMio === "number");
+    const visible = $derived(showNeg || showEinsman);
+
+    const yearLabel = $derived(new Date().getFullYear().toString());
+</script>
+
+{#if visible}
+    <div class="flex flex-col gap-3">
+        <header class="flex flex-col gap-0.5">
+            <h4 class="text-sm font-semibold text-stein-800">
+                Überschussstrom & Abregelung {yearLabel}
+            </h4>
+            <p class="m-0 text-[11px] text-stein-600">
+                Hohe Wind- und Solarspitzen drücken den Strompreis, bei
+                Netzengpässen werden Anlagen abgeregelt — und entschädigt.
+                Beides Folgekosten der bisherigen Zubaupolitik.
+            </p>
+        </header>
+
+        <div class="grid gap-2 sm:grid-cols-2">
+            {#if showNeg && negHoursYtd != null}
+                <div class="rounded-xs border-l-4 border-fire-400 bg-white px-3 py-2.5 shadow-sm">
+                    <div class="flex items-baseline gap-1.5">
+                        <span class="text-2xl font-bold tabular-nums text-stein-900">
+                            {fmtNum(negHoursYtd)}
+                        </span>
+                        <span class="text-xs text-stein-600">Stunden</span>
+                    </div>
+                    <span class="mt-0.5 block text-[11px] leading-snug text-stein-600">
+                        mit negativem Day-Ahead-Preis
+                        <span class="text-stein-400">(DE-LU)</span>
+                    </span>
+                </div>
+            {/if}
+
+            {#if showEinsman && einsmanEurMio != null}
+                <div class="rounded-xs border-l-4 border-fire-400 bg-white px-3 py-2.5 shadow-sm">
+                    <div class="flex items-baseline gap-1.5">
+                        <span class="text-2xl font-bold tabular-nums text-stein-900">
+                            {fmtNum(einsmanEurMio, 0)}
+                        </span>
+                        <span class="text-xs text-stein-600">Mio €</span>
+                    </div>
+                    <span class="mt-0.5 block text-[11px] leading-snug text-stein-600">
+                        Entschädigung Abregelung (§&nbsp;13 EnWG, §&nbsp;51 EEG)
+                        {#if einsmanAsOf}
+                            <span class="block text-[10px] text-stein-400">
+                                Stand {einsmanAsOf}
+                            </span>
+                        {/if}
+                    </span>
+                </div>
+            {/if}
+        </div>
+    </div>
+{/if}