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/block-types.ts

@@ -362,6 +362,21 @@ export interface LiveStrommixBlockData {
   disclaimer_threshold_eur_mwh?: number;
   /** Reference (resolved: { _slug, _type, ...page-fields } or just slug string). */
   explanation_page?: string | { _slug?: string; title?: string };
+  // ---------------------------------------------------------------------
+  // Stammdaten (BNetzA Marktstammdatenregister, manuell gepflegt). Treiben
+  // den Live-vs-Installiert-Auslastungs-Bereich. Stand quartalsweise
+  // aktualisieren — die Werte bewegen sich um 1–3 GW pro Quartal.
+  // ---------------------------------------------------------------------
+  installed_wind_onshore_gw?: number;
+  installed_wind_offshore_gw?: number;
+  installed_solar_gw?: number;
+  /** Free-form Stand-Marker für die Stammdaten, z. B. "2025-Q4". */
+  installed_as_of?: string;
+  /** Optional: Hand-gepflegte EinsMan-Entschädigungssumme YTD (Mio €).
+   *  BNetzA liefert das nur quartalsweise per PDF. Wenn nicht gesetzt,
+   *  zeigt das Widget den Counter ohne Eurosumme. */
+  einsman_costs_ytd_eur_mio?: number;
+  einsman_costs_as_of?: string;
   layout?: BlockLayout;
 }