fix(scheduling): only show candidates that can actually qualify after swap

replace_qualified (换下):
- Exclude already-qualified inventory (totalMileage >= yearTarget, gap=0)
- Only keep candidates where canQualifyAfterSwap=true
- Skip suggestions with no qualifiable candidates (e.g., too few days left)
- Reason text now shows customer's remaining capacity: "日均 318km × 53天 ≈ 1.7万km"

Before: showed 粤AGP9738 (缺口 0, already at target) — pointless
After: shows 粤AGQ5808 (缺口 1.7万, 换后 3.0万, 可达标) — meaningful

All replace_qualified candidates now guaranteed canQualifyAfterSwap=true.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/server/routes/scheduling/algorithm.ts

@@ -144,15 +144,21 @@ export function generateSuggestions(
   }
 
   // --- replace_qualified (medium priority) ---
+  // Swap out the qualified car, swap in a car that NEEDS mileage.
+  // The high-mileage customer will drive it hard → helps it reach target.
+  // Exclude candidates already at target (gap <= 0) — swapping those in is pointless.
   for (const vehicle of qualified) {
     if (vehicle.customerAvgDaily <= vehicle.dailyRequiredMileage) continue;
 
     const candidates: CandidateVehicle[] = inventoryVehicles
-      .filter(
-        (inv) =>
-          isTypeCompatible(vehicle.vehicleType, inv.vehicleType) &&
-          inv.region === vehicle.region,
-      )
+      .filter((inv) => {
+        if (!isTypeCompatible(vehicle.vehicleType, inv.vehicleType)) return false;
+        if (inv.region !== vehicle.region) return false;
+        // Must still need mileage — exclude already-qualified inventory
+        const effectiveTarget = inv.yearTarget ?? vehicle.yearTarget;
+        if (effectiveTarget > 0 && inv.totalMileage >= effectiveTarget) return false;
+        return true;
+      })
       .map((inv) => {
         const effectiveTarget = inv.yearTarget ?? vehicle.yearTarget;
         const mileageGap = Math.max(0, effectiveTarget - inv.totalMileage);
@@ -175,15 +181,23 @@ export function generateSuggestions(
         };
       })
       .sort((a, b) => {
+        // 1. canQualifyAfterSwap first
         if (a.canQualifyAfterSwap !== b.canQualifyAfterSwap)
           return a.canQualifyAfterSwap ? -1 : 1;
+        // 2. Among qualifiable: biggest gap first (most value from the swap)
         return b.mileageGap - a.mileageGap;
       })
+      // Only keep candidates that can actually qualify at this customer
+      .filter(c => c.canQualifyAfterSwap)
       .slice(0, 5);
 
+    // Skip if no candidate can reach target — swap would be pointless
+    if (candidates.length === 0) continue;
+
     const yearRate = vehicle.yearTarget > 0 ? Math.round((vehicle.currentYearMileage / vehicle.yearTarget) * 100) : 0;
-    const reason = `该车在客户「${vehicle.customer}」处已达标（完成率 ${yearRate}%），客户日均 ${Math.round(vehicle.customerAvgDaily)} km，属于高里程客户。`
-      + `\n建议：将此车换下，换上一辆里程少的车，利用该客户的高日均里程帮助新车快速达标。`;
+    const canAddKm = vehicle.customerAvgDaily * vehicle.daysLeft;
+    const reason = `该车在客户「${vehicle.customer}」处已达标（完成率 ${yearRate}%），客户日均 ${Math.round(vehicle.customerAvgDaily)} km × ${vehicle.daysLeft} 天 ≈ ${fmtKmSimple(canAddKm)} km。`
+      + `\n建议：换上里程未达标的车，利用该客户的高日均帮新车快速冲线。`;
 
     suggestions.push({
       id: `qualified-${vehicle.plateNumber}`,