fix(scheduling): enforce one active intervention per current vehicle

Business rule: a running vehicle can hold AT MOST ONE active (sent|executed)
intervention. Switching to a different candidate requires cancelling the
prior one first.

- Server: insertNotification dedup key changes from (suggestion_id,
  candidate_plate) to just suggestion_id; 409 response includes the blocking
  candidate plate
- Detail modal: shows a banner naming the locked candidate; non-active
  candidates render a disabled "该车已有其他干预，请先解除" hint instead
  of the action button
- Batch: pickBestCandidate returns null for any suggestion already holding
  an active intervention — the whole suggestion is excluded

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/modules/scheduling/SchedulingModule.tsx

@@ -147,10 +147,13 @@ function SkeletonPage() {
 }
 
 function pickBestCandidate(s: SchedulingSuggestion): CandidateVehicle | null {
-  // Prefer a candidate that can qualify and isn't already notified
-  const available = s.candidates.filter(c => !c.notificationStatus || c.notificationStatus === 'cancelled');
-  if (available.length === 0) return null;
-  return available.find(c => c.canQualifyAfterSwap) ?? available[0];
+  // Business rule: at most one active intervention per suggestion. If ANY
+  // candidate is already intervened, skip the whole suggestion in batch flow.
+  const hasActive = s.candidates.some(
+    c => c.notificationStatus === 'sent' || c.notificationStatus === 'executed',
+  );
+  if (hasActive) return null;
+  return s.candidates.find(c => c.canQualifyAfterSwap) ?? s.candidates[0] ?? null;
 }
 
 export default function SchedulingModule() {