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/strommix.ts

@@ -50,4 +50,62 @@ export type StrommixResponse = {
   staleStrommix: boolean;
   staleCrossborder: boolean;
   stalePrice: boolean;
+
+  // ---------------------------------------------------------------------
+  // Historical aggregates (sourced from CMS `_history` endpoint over
+  // raw upstream snapshots; null = no data yet for this widget).
+  // ---------------------------------------------------------------------
+
+  /** Today's residual-load curve. One entry per upstream slot (typically
+   *  15 min). `loadMw - weatherRenewableMw` = the gap conventional plants
+   *  + imports must fill. `null` when history backend is empty / disabled. */
+  todayHourly: ResidualLoadPoint[] | null;
+
+  /** Negative-price hours year-to-date (DE-LU, day-ahead). Counted by
+   *  iterating history snapshots and summing 15-min slots whose price
+   *  was below 0. */
+  negPriceHoursYtd: number | null;
+
+  /** Cross-border net balance YTD in GWh (positive = net export from DE,
+   *  negative = net import). */
+  netCrossborderGwhYtd: number | null;
+
+  /** Per-country YTD balance, same sign convention as `netFlowGw`. */
+  crossborderByCountryGwhYtd: Record<string, number> | null;
+
+  /** Cross-border balance for *today only*, GWh per country and total.
+   *  Same sign convention. `null` when crossborder history is empty. */
+  crossborderTodayGwh: { total: number; perCountry: Record<string, number> } | null;
+
+  /** Most recent contiguous run of `(wind+solar) / load < 10%`. `null`
+   *  when no such run exists in retained history. `endedHoursAgo: 0`
+   *  means the run is still ongoing (last slot is within the threshold). */
+  lastDunkelflaute: DunkelflautePhase | null;
+
+  /** Top 5 longest Dunkelflaute phases in retained history, newest first
+   *  if equal length. Empty array when history exists but no qualifying
+   *  phase found. `null` when history is unavailable. */
+  worstDunkelflauten: DunkelflautePhase[] | null;
+};
+
+/** One upstream slot in the residual-load series. */
+export type ResidualLoadPoint = {
+  /** Unix seconds at the start of the slot. */
+  unix: number;
+  /** Total electrical load on the German grid (MW). */
+  loadMw: number;
+  /** Wind onshore + wind offshore + solar (MW). */
+  weatherRenewableMw: number;
+};
+
+/** A contiguous low-renewables run, computed from history snapshots. */
+export type DunkelflautePhase = {
+  /** Unix sec of the first slot below the threshold. */
+  startUnix: number;
+  /** Run duration in hours (slot count × slot length). */
+  durationH: number;
+  /** Lowest Wind+Solar share observed during the run, percentage of load. */
+  minPct: number;
+  /** Hours since the run ended (0 = ongoing). */
+  endedHoursAgo: number;
 };