fix(meteo): fall back to nearest cached data when the history API fails

The dashboard widget lost the previous-year temperature and the difference:
when the day's previous-year row is not yet cached and the Visual Crossing
history call fails (quota exhausted, outage, or a plan without history
access), the route silently returned previousYear: null and the widget
hid the comparison.

The weather cache accumulates one previous-year row per day, so the route
now falls back to the nearest cached previous-year entry within 7 days of
the weekday-aligned target date, keeping the comparison on screen through
API failures. The exact API error remains logged server-side
(🌤️ [ERROR] / [FETCH-HISTORY]) for diagnosis.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FHdE9nEh8zHhQNGpCgrtYe

server/routes.ts

@@ -5723,6 +5723,16 @@ RÉSUMÉ DU SCAN
         }
       }
 
+      // Repli : si l'API historique a échoué (quota épuisé, panne, plan sans
+      // accès à l'historique), reprendre la ligne en cache la plus proche de
+      // la date cible plutôt que de faire disparaître la comparaison N-1
+      if (!previousYearData) {
+        previousYearData = await storage.getNearestWeatherData(previousYearDate, false, 7);
+        if (previousYearData) {
+          console.log(`🌤️ [FALLBACK] Using nearest cached previous-year data (${previousYearData.date}) for target ${previousYearDate}`);
+        }
+      }
+
       // Toujours retourner une réponse même si seule l'année actuelle est disponible
       const response = {
         currentYear: currentYearData ? {