Add 24-month dept split chart below rolling trend

Stacked bar chart (Rooms/Dry/Wet) for the previous 12 months and last
12 months side by side, with a reference line marking the boundary.
No extra API calls — data comes from the existing 36-month fetch.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/routes/weekly-actual.js

@@ -253,6 +253,24 @@ export async function weeklyActualRoutes(fastify) {
       })
     }
 
+    // Monthly dept split for last 24 months (indices 12–35)
+    // — previous 12 months followed by last 12 months
+    const monthly_split = []
+    for (let i = 12; i < 36; i++) {
+      const tm   = trendMonths[i]
+      const data = trendRevRaw[i]?.data ?? []
+      let rooms = 0, dry = 0, wet = 0
+      for (const d of data) {
+        rooms += parseFloat(d.accom?.otb ?? 0)
+        dry   += parseFloat(d.dry?.otb   ?? 0)
+        wet   += parseFloat(d.wet?.otb   ?? 0)
+      }
+      monthly_split.push({
+        label: `${MONTH_LABELS[tm.month - 1]}-${String(tm.year).slice(2)}`,
+        rooms, dry, wet,
+      })
+    }
+
     return {
       week_ending: toDateStr(weekEndDate),
       week_start:  toDateStr(weekStartDate),
@@ -262,6 +280,7 @@ export async function weeklyActualRoutes(fastify) {
       occupancy,
       month_progress,
       monthly_trend,
+      monthly_split,
     }
   })
 }