fix(strommix): per-field fallback to Energy-Charts when SMARD value missing

The widget rendered "Keine gültigen Live-Werte" the moment SMARD's
load filter (id 410) lagged behind the production filters by even one
slot — the aggregator coalesced "filter unavailable" with "filter
returned 0" and the percent calculation refused to divide by zero.

Three changes:

1. SmardSnapshot.values now uses `number | null` so callers can tell
   "filter genuinely missing" apart from "filter reported zero" (e.g.
   solar at night, decommissioned nuclear).
2. Aggregator pulls Energy-Charts (CMS strommix_de) in parallel and
   uses it as a per-field fallback for any SMARD null. New
   `dataSource: "smard+energy-charts"` flag travels in the response so
   the attribution stays honest. Pure SMARD when everything resolves;
   pure Energy-Charts when SMARD is entirely down; the hybrid label
   for the common case where only load drops out.
3. Widget renders the production breakdown + context row + price row
   even when the percentage is unavailable, with a clear "Lastwert
   von SMARD gerade nicht verfügbar" hint instead of going blank.

src/lib/components/blocks/StrommixBlock.svelte

@@ -186,18 +186,25 @@
       </div>
     {:else if !live}
       <div class="p-6 text-center text-neutral-500 text-sm">Lade Live-Daten…</div>
-    {:else if renewablePct == null}
-      <div class="p-6 text-sm text-amber-700">Keine gültigen Live-Werte verfügbar.</div>
     {:else}
       <div class="p-6 text-center">
-        <div class="text-6xl sm:text-7xl font-extrabold tabular-nums {accentClasses.pct}">
-          {Math.round(renewablePct)}%
-        </div>
-        <div class="mt-1 text-sm text-neutral-700">Wind &amp; Solar am Bedarf</div>
-        <div class="mt-3 text-base font-semibold {accentClasses.label}">→ {statusLabel}</div>
-        {#if argumentHtml}
-          <div class="prose prose-sm mt-3 max-w-prose mx-auto text-neutral-800">
-            {@html argumentHtml}
+        {#if renewablePct != null}
+          <div class="text-6xl sm:text-7xl font-extrabold tabular-nums {accentClasses.pct}">
+            {Math.round(renewablePct)}%
+          </div>
+          <div class="mt-1 text-sm text-neutral-700">Wind &amp; Solar am Bedarf</div>
+          <div class="mt-3 text-base font-semibold {accentClasses.label}">→ {statusLabel}</div>
+          {#if argumentHtml}
+            <div class="prose prose-sm mt-3 max-w-prose mx-auto text-neutral-800">
+              {@html argumentHtml}
+            </div>
+          {/if}
+        {:else}
+          <!-- Production data arrived but load is missing → no percentage,
+               but we still show what we have so the widget isn't blank. -->
+          <div class="text-sm text-amber-700">
+            Lastwert von SMARD gerade nicht verfügbar — Prozentanzeige ausgesetzt.
+            Erzeugung wird unten in der Detailansicht weiter angezeigt.
           </div>
         {/if}
       </div>
@@ -270,9 +277,14 @@
         <div class="text-neutral-500 font-semibold">Last</div><div class="font-semibold">{fmtMw(live.loadMw)}</div>
       </div>
       <p class="mt-1 text-[11px] text-neutral-500">
-        Quelle: {live.dataSource === "smard"
-          ? "smard.de (Bundesnetzagentur)"
-          : "energy-charts.info (ENTSO-E + SMARD)"}
+        Quelle:
+        {#if live.dataSource === "smard"}
+          smard.de (Bundesnetzagentur)
+        {:else if live.dataSource === "smard+energy-charts"}
+          smard.de + energy-charts.info (Last-Wert ergänzt)
+        {:else}
+          energy-charts.info (ENTSO-E + SMARD)
+        {/if}
         — 15-Min-Auflösung, Stand:&nbsp;{fmtTime(live.measuredAtUnix)}&nbsp;Uhr.
         Konventionell = Braunkohle + Steinkohle + Erdgas + Kernkraft (Pumpspeicher,
         Sonstige Konventionelle und Wasserkraft sind separat ausgewiesen).

src/lib/smard.ts

@@ -116,8 +116,12 @@ export type SmardSnapshot = {
    *  set, so every value is from a slot that all filters have reported.
    *  Unix seconds (not ms). */
   measuredAtUnix: number;
-  /** Average MW for each filter at the aligned slot. Missing filters → 0. */
-  values: Record<SmardFilter, number>;
+  /** Average MW for each filter at the aligned slot. `null` means the
+   *  filter could not be resolved (404, all-null series, fetch error)
+   *  — distinguished from a genuine zero so callers can choose a
+   *  fallback source per-field instead of ignoring real zeroes
+   *  (e.g. solar at night, nuclear in DE). */
+  values: Record<SmardFilter, number | null>;
 };
 
 /**
@@ -174,9 +178,9 @@ export async function fetchSmardSnapshot(
     })
   );
 
-  const values = {} as Record<SmardFilter, number>;
+  const values = {} as Record<SmardFilter, number | null>;
   for (const key of Object.keys(SMARD_FILTERS) as SmardFilter[]) {
-    values[key] = 0;
+    values[key] = null;
   }
   for (const [key, mwh] of aligned) {
     if (mwh != null) {

src/lib/strommix.ts

@@ -11,10 +11,13 @@ export type StrommixResponse = {
   // the rest of the values. Cross-border + prices may have their own,
   // earlier or later, timestamps.
   measuredAtUnix: number | null;
-  /** Where production + load came from. `smard` is canonical (German
-   *  grid feed); `energy-charts` is the fallback when SMARD is
-   *  unreachable. */
-  dataSource: "smard" | "energy-charts";
+  /** Where production + load came from. `smard` = pure SMARD (canonical);
+   *  `smard+energy-charts` = SMARD with Energy-Charts as per-field
+   *  fallback for filters SMARD couldn't resolve at the snapshot's slot
+   *  (typically the load filter when SMARD's pipeline lags more than
+   *  the production filters); `energy-charts` = SMARD entirely
+   *  unreachable, full fallback. */
+  dataSource: "smard" | "smard+energy-charts" | "energy-charts";
   // Production (MW)
   windOnshoreMw: number;
   windOffshoreMw: number;

src/routes/api/strommix/+server.ts

@@ -24,7 +24,7 @@
 import type { RequestHandler } from "./$types";
 import { env } from "$env/dynamic/public";
 import type { StrommixResponse } from "$lib/strommix";
-import { fetchSmardSnapshot, type SmardSnapshot } from "$lib/smard";
+import { fetchSmardSnapshot } from "$lib/smard";
 
 const CMS_BASE = (
   env.PUBLIC_CMS_URL ||
@@ -32,6 +32,20 @@ const CMS_BASE = (
   "http://localhost:3000"
 ).replace(/\/$/, "");
 
+type StrommixRaw = {
+  wind_onshore_mw?: number | null;
+  wind_offshore_mw?: number | null;
+  solar_mw?: number | null;
+  biomass_mw?: number | null;
+  hydro_mw?: number | null;
+  lignite_mw?: number | null;
+  hard_coal_mw?: number | null;
+  gas_mw?: number | null;
+  nuclear_mw?: number | null;
+  load_mw?: number | null;
+  measured_at_unix?: number | null;
+};
+
 type CrossborderRaw = {
   flow_at_gw?: number | null;
   flow_be_gw?: number | null;
@@ -113,26 +127,53 @@ async function fetchPriceAtNow(
 }
 
 export const GET: RequestHandler = async ({ fetch }) => {
-  const [smard, cross, priceDe, priceFr] = await Promise.all([
+  const [smard, energyCharts, cross, priceDe, priceFr] = await Promise.all([
     fetchSmardSnapshot(fetch),
+    // Energy-Charts via the CMS — used as per-field fallback when SMARD
+    // returns null for a specific filter (notably load 410 when SMARD's
+    // pipeline lags more than the production filters).
+    fetchOne<StrommixRaw>("strommix_de", fetch),
     fetchOne<CrossborderRaw>("crossborder_de", fetch),
     fetchPriceAtNow("DE-LU", fetch),
     fetchPriceAtNow("FR", fetch),
   ]);
 
-  // Production breakdown — SMARD as primary. Each filter is in MW
-  // already (SMARD MWh × 4).
-  const v: SmardSnapshot["values"] = smard?.values ?? {
-    lignite: 0, hardCoal: 0, gas: 0, nuclear: 0,
-    windOnshore: 0, windOffshore: 0, solar: 0,
-    hydro: 0, biomass: 0,
-    otherConventional: 0, pumpedStorage: 0, load: 0,
+  const ec = energyCharts.data ?? {};
+
+  // Per-field fallback. SMARD value `null` = filter unavailable; we
+  // reach for the Energy-Charts equivalent. SMARD `0` is preserved
+  // (genuine night-time solar / decommissioned nuclear / etc).
+  const pick = (smardVal: number | null | undefined, ecVal: number | null | undefined): number =>
+    typeof smardVal === "number"
+      ? smardVal
+      : typeof ecVal === "number" && Number.isFinite(ecVal)
+        ? ecVal
+        : 0;
+
+  const v = {
+    lignite: pick(smard?.values.lignite, ec.lignite_mw),
+    hardCoal: pick(smard?.values.hardCoal, ec.hard_coal_mw),
+    gas: pick(smard?.values.gas, ec.gas_mw),
+    nuclear: pick(smard?.values.nuclear, ec.nuclear_mw),
+    windOnshore: pick(smard?.values.windOnshore, ec.wind_onshore_mw),
+    windOffshore: pick(smard?.values.windOffshore, ec.wind_offshore_mw),
+    solar: pick(smard?.values.solar, ec.solar_mw),
+    hydro: pick(smard?.values.hydro, ec.hydro_mw),
+    biomass: pick(smard?.values.biomass, ec.biomass_mw),
+    load: pick(smard?.values.load, ec.load_mw),
   };
 
+  // Track whether any field needed the Energy-Charts fallback so the
+  // widget can attribute the source honestly.
+  const usedFallback =
+    !smard ||
+    (Object.keys(v) as (keyof typeof v)[]).some(
+      (k) => smard.values[k as keyof typeof smard.values] == null && (v[k] ?? 0) > 0
+    );
+
   // Conventional, the way the widget label spells it: lignite + hard
   // coal + gas + nuclear. Pumped-storage + biomass + run-of-river
-  // hydro are NOT included (different categories or already-stored
-  // energy that would double-count).
+  // hydro are NOT included.
   const conventionalMw = v.lignite + v.hardCoal + v.gas + v.nuclear;
   const weatherRenewableMw = v.windOnshore + v.windOffshore + v.solar;
 
@@ -153,8 +194,8 @@ export const GET: RequestHandler = async ({ fetch }) => {
   const netFlowGw = Object.values(flowsByCountry).reduce((a, b) => a + b, 0);
 
   const body: StrommixResponse = {
-    measuredAtUnix: smard?.measuredAtUnix ?? null,
-    dataSource: smard ? "smard" : "energy-charts",
+    measuredAtUnix: smard?.measuredAtUnix ?? ec.measured_at_unix ?? null,
+    dataSource: !smard ? "energy-charts" : usedFallback ? "smard+energy-charts" : "smard",
     windOnshoreMw: v.windOnshore,
     windOffshoreMw: v.windOffshore,
     solarMw: v.solar,
@@ -173,7 +214,7 @@ export const GET: RequestHandler = async ({ fetch }) => {
     priceDeEurMwh: priceDe.price,
     priceFrEurMwh: priceFr.price,
     priceMeasuredAtUnix: priceDe.slotUnix,
-    staleStrommix: smard == null,
+    staleStrommix: smard == null && !energyCharts.data,
     staleCrossborder: cross.stale,
     stalePrice: priceDe.stale || priceFr.stale,
   };