feat: time-aware Super Contexte synthesis

- knowledge.py: classify trades by maturity before building synthesis
  prompt; only mature trades (≥35% elapsed) contribute to P&L stats
  and conclusions; immature trades listed for transparency only
- Add 6h staleness gate on POST /synthesize (force=true to override)
- System prompt now includes hard timing rule: GPT-4o must not revise
  existing conclusions because of newly-added immature trades
- useApi.ts: useSynthesizeKnowledge accepts force boolean param
- SuperContexte.tsx: shows amber notice with age when skipped + offers
  "Force quand même" button; success banner uses new response shape

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

frontend/src/hooks/useApi.ts

@@ -526,11 +526,14 @@ export const useKnowledgeEntries = () =>
 export const useSynthesizeKnowledge = () => {
   const qc = useQueryClient()
   return useMutation({
-    mutationFn: () => api.post('/knowledge/synthesize').then(r => r.data),
-    onSuccess: () => {
-      qc.invalidateQueries({ queryKey: ['knowledge-state'] })
-      qc.invalidateQueries({ queryKey: ['knowledge-history'] })
-      qc.invalidateQueries({ queryKey: ['knowledge-entries'] })
+    mutationFn: (force: boolean = false) =>
+      api.post(`/knowledge/synthesize${force ? '?force=true' : ''}`).then(r => r.data),
+    onSuccess: (_data) => {
+      if (!_data?.skipped) {
+        qc.invalidateQueries({ queryKey: ['knowledge-state'] })
+        qc.invalidateQueries({ queryKey: ['knowledge-history'] })
+        qc.invalidateQueries({ queryKey: ['knowledge-entries'] })
+      }
     },
   })
 }